Remember that only the older revisions of the p5a run well with a k6-2+ or k6-3+ (if you want to run a + cpu on it). 1.05 and 1.06 will run them slow. No problems with the normal k6-3.
Can your fic board cache 256mb? Otherwise 128mb will be a better choice for max. performance (same on the p5a, but no issue when using k6-3) . Your board has ''one of them agp slots'' but a banshee should be fine. V3 will be to much.
For oldest dos glide games i would stick a voodoo1 in your dos system. It can have troubles with fast p3 systems but it will work fine on even the fastest k6.
asus tx97-e, 233mmx, voodoo1, s3 virge ,sb16
asus p5a, k6-3+ @ 550mhz, voodoo2 12mb sli, gf2 gts, awe32
asus p3b-f, p3-700, voodoo3 3500TV agp, awe64
asus tusl2-c, p3-S 1,4ghz, voodoo5 5500, live!
asus a7n8x DL, barton cpu, 6800ultra, Voodoo3 pci, audigy1