What "pure DOS games" do you want to run?
ISA is very very useful for DOS, and there's not much out there that won't run fast on a P3-600, so you can ditch the P4.
As for choosing between the P3 and 486, it depends on the target games. The P3 is much faster and probably does not have a 'turbo' function. You can disable caches, but it will always run several times faster than the 486. If you want >1994 games, that's generally an advantage. If you want <1994 games, the 486 is already very fast and things might get unplayable or worse, games would crash and audio would glitch out due to timing issues on the P3.
For reference, I've built a 486SX33 with turbo (=slowdown) function for old games and a K6-2 350 for new DOS games. Even at 350MHz (and 64MB RAM) I'm hitting stability problems which may be clock speed or RAM size related.
If I had to choose one system for the largest number of DOS games, I'd go for the 486. But if you are less interested in Monkey Island and Ultima 6/7 than in Quake and Descent, the P3-600 might be better.
Also, not all ISA sound cards are equal or equally desirable. Which do those systems have?
P.s. 2GB on P3? There are a few board that allowed it, but it's highly unusual. Expect problems with DOS >512MB, Win98SE too unless you patch it. Maybe worth double-checking that figure. If it is true, the board might be very exotic (Serverworks chipset...) which could introduce other complications. In any event, there is no reason whatsoever to have that much RAM for any pure DOS system. I find 64MB to be the sweet spot for later DOS games, which is more 'normal' on a P3 anyway: a single 64MB PC100 DIMM.
