I'll chime in with my 2 cents on this topic. I just recently built two BX motherboard based PC's for my newer DOS/older Win9x games. I have a post on Vogons with my specs for those computers. The nice thing about the ASUS P2B boards is they have great support for the Slotkets or Powerleap slot converters. See here http://homepage.hispeed.ch/rscheidegger/p2b_p … pgrade_faq.html
I have two such PC's. One a 1Ghz Coppermine and one a 1.4Ghz Tualatin.
Neither of these boxes are my main rig. I have an Athlon 64 for that, which will become a Core2Duo eventually. As much as I like my games to live in the past, I do have to keep up and live with the present.
IMO, most games fall into three categories.
1) Old DOS stuff that runs best on an old PC, with an ISA/VLB video card like a Tseng ET4000, Paradise, Cirrus Logic, S3, etc. A SB16 is a must.
2) Newer era DOS games and old Win9x *only* games that don't run on 2000 or XP. Most Glide and older DirectX 3-6 games fall into here. SB16 is okay, but a SB AWE64, Aureal or some PCI card with EAX/A3D is better.
3) Anything written that works on both 9x or XP, XP only, needs 3D, DirectX 9, all new games, OpenGL, etc. Generally the fastest PC you can afford.
Unless you have the hardware laying around for #1, it doesn't make sense to build one. Of course if this is your hobby and passion, then money is no object anyway so you'll rightly ignore my post. I tend to think only #2 and #3 are necessary, since #1 games can be run in DOSBox on #3. DOSBox will only get better. I have a lot faith in the developers. They may even break into some #2 DOS games assuming your #3 is fast enough.
Knowing that, I would concentrate on building a #2 if money was a factor. If you have the time, space and money to kill, then you wouldn't need to post here and you'd have all three. eBay is your friend.
-Robert