I'm no Tom's Hardware or Anandtech, and don't have access to all the wealth of hardware that's out there. Contributions and information to fill out this section would is welcome. This stuff changes very often, and peoples' experience with hardware would be useful.
If you're using Linux, you must be smart enough to know that there isn't a plain answer to this question. There seem to be 3 choices for hardware accelerated 3D these days:
3dfx: Voodoo cards
According to Tom's Hardware and Anadtech, the Radeon is king when playing at very high resolution (as in 1600x1200), at 32bpp, in Windows. Otherwise the GeForce is king. There are two problems with this. We don't normally play at 1600x1200/32bb, and we don't play on Windows (at least I don't).
There aren't many recent video card benchmarks out for Linux. The most recent one I've seen is from March 2001 at http://www.linuxhardware.org/features/01/03/19/0357219.shtml. Considering the dearth of benchmarks out there, this needs to be taken as a canonical benchmark, so I simply quote their conclusion:
At this point the performance numbers tell a pretty simple story, if it's raw speed you are looking for, the GeForce 2 is your choice. There is very little performance drawback to running your favorite games in Linux instead of Windows with this card. It provides a truly impressive performace across the board. The Radeon's performance will almost definitely improve as the DRI drivers mature, but for now, especially for the impatient, it is simply not a good choice for the hard core 3d gamer.
If, however, you are a graphics designer, and want a card with impeccable 2d image quality, with 3d graphics only a secondary priority, the Radeon is your best bet. The DRI drivers, even in their current state, are quite usable. For 2d only users, XFree86 4.0.2 provides production quality 2d drivers. The GeForce thoroughly trounced the Radeon in the Xmark performance test, so if you aren't running at a ultra high resolution, or aren't that picky, the GeForce is once again a better pick.
Now for my own input. The Radeon is a pretty amazing card. It's what I use, and I have yet to see a game that needs more power than the Radeon is able to provide. However, the OpenGL renderer for the Radeon is buggy, although the only games I've seen that suffer greatly are Loki Software's Heavy Metal and Soldier Of Fortune. Hopefully the people doing Mesa for the Radeon will fix this very soon since the Radeon is the best option for people who don't want to rely on the closed source, proprietary GeForce. As of June 2002, SVGAlib support Radeon cards is shaky. Developers have reported that SVGAlib works on the Radeon 7500, Radeon QD (ddr 64MB model) but has problems on the Radeon VE.
Now about the Voodoo cards. Unfortunately, 3dfx was bought out by nVidia, so these cards are a dead end market. If you're out to play the bleeding edge games like Rune or Tribes2, you'll want the Voodoo 3, 4 or 5. Preferably the 4 or 5. I think the Voodoo 5 is basically a Voodoo 4 with a second processer. However, this processor is not utilized by the Linux driver, and rumor says that the Linux 3dfx driver will never support it. So as far as Linux is concerned, the Voodoo 4 and 5 are the same card. All the drivers, Glide2 library and OpenGL renderers for the Voodoo cards were open sourced by 3dfx before they when under. It is an embarrasment to the Linux and open source community in general that this company failed. SVGAlib officially supports only the Voodoo Banshee and the Voodoo III, but from first hand experience, I've seen SVGAlib programs run on all the Voodoo cards.
Now that Linux is beginning to mature, this question isn't as important as it used to be. Once upon a time, soundcards without onboard MIDI chips (most PCI sound cards) didn't do MIDI. This was mostly a problem for things like xdoom or lxdoom using musserv. These days we have MIDI emulators like Timidity and libraries like SDL which don't require hardware MIDI support. Frankly, I've had many cards and I can't tell the difference between any of them for gaming. If you want to do things like convert a record LP to digital format, then your choice of a soundcard with a professional grade A/D converter is absolutely crucial. For this HOWTO, we'll assume that you're more of a gamer than a studio recording engineer.
Your decision should be based on what will be the easiest to configure. If you already have a card and it works well, that's good enough. If you're in the market to buy a sound card, get something that will take you a second to configure. PCI cards are much easier to deal with than ISA since you don't need to tell their drivers about which system resources (IRQ, DMA, I/O addresses) to use. Some ISA cards ARE plug-n-play, like the Creative AWE-64, and the Linux kernel has come a long way in auto configuring them.
My personal recommendation is any card which has the es1370 or es1371 chip, which uses the es1370 and es1371 sound drivers on Linux. These cards include the older Ensoniq es1370 and newer Creative PCI-128. These cards are extremely cheap and trivial to get working under Linux.
I used to be quite a big fan of the Creative Soundblaster AWE 32, AWE 64 and AWE 64 gold soundcards. They are ISA, but are plug-n-play. A couple of issues to note. First, the Creative AWE HOWTO is very out of date. Second, AFAIK, Creative never released a Linux driver that uses the AWE 64's extra 32 voices (and they never released programming information for it either). So to a Linux users, the AWE 64 and 32 are nearly identical sound cards. If anyone has more information about the differences that a Linux user would see between the AWE 64 and 32, I'd like to hear from you.
The Creative Soundblaster Live! is an extremely popular PCI sound card these days. I've never owned one, so I cannot comment here. However, there have been numerous reports about serious problems with the Live! and AMD motherboards that use the 686b southbridge. A google search should turn up alot of information on this problem.
A more relevent issue is speakers, but even here the difference isn't huge. I've had expensive Altec Lansing speakers perform only slightly better than el-cheapo speakers. You get what you pay for with speakers, but don't expect a huge difference. You'll want to get something with a separate sub-woofer; this does make a difference at a cost of extra power and connector wires.