Not much to add, other than you may be more interested in easy underclocking, rather than overclocking. Try to find a board with BIOS-controlled FSB, so that you can easily drop your 600MHZ down to below 450. There are some DOS games that don't work on 450+ but are quite fine at 300.
Also, if you can, get a board with SBLink. This would allow using a PCI sound card in DOS as if it was a "proper" DOS era card, with its own IRQ, DMA, etc. Naturally, that would imply the card has to support it as well. Alternatively, get an ISA-compatible motherboard. That said, unless you want to risk it, stay away from ALi or VIA chipsets, go Intel. Don't remember which, but there were some that has ISA slots, yet the way those were connected meant DOS wouldn't see anything on them. In other words, 440BX is your best bet.
If you do go the PCI and SBLink way, a fine choice is YMF724/744-based cards. Most have good SNR, quite a few have SBLink, and every single one supports SB Pro standard. So for DOS that is awesome, while for Windows, unless you want 3D effects, anything is fine. For 3D, however, you have two options: something Creative with EAX, or Aureal A3D (Vortex II recommended, as it's newer than Vortex 1, yet being 1998, still fits in your time frame). You could even go with two cards. Vortex cards are still reasonably affordable, but beware eBay. Some folks try to sell C-Media as Aureal by relabeling the chip. Not that there is anything wrong with C-Media (it has it's own 3D, which can emulate EAX and A3D, to a reasonable extent), but it doesn't (at least, shouldn't) cost 60+ USD that Aureal usually sells for. Also, YMF-based cards can also emulate both EAX and A3D, so, unless you want the real thing, you don't want a second card. Note that in either case emulation isn't perfect.
Another warning, though: YMF cards don't support some compression formats for PCM sounds, notably, ADPCM Duke Nukem uses. But there are very few games that do. I'm not going into detal regarding DOS, because if that was your thing, you wouldn't be building a P3, and it is a whole another can of worms regarding sound. But if you are interested, I am sure there are plenty of people here that could tell you everything there is to it, including stuff Ensoniq or Yamaha themselves did not know 😀
Regarding GPU, just two questions: 1) what games do you plan to play, and 2) how important is DOS compatibility? (how far down the rabbit hole do you plan to go with DOS)
In any case, peruse this chart to make an informed decision.