Regarding ram: When I build my retro P3-1000 for win98, I started out with 512 MB, as this is the max for the release version of the OS. When I started to install games, I soon downgraded the ram to 384 and then to 256 MB. The reason was that games would not install. After lengthy online searches, I found the info, took out the ram and voilá, it worked. Considering the time spend to find these issues, I decided to leave ram at 256MB for the win98se machine. If the game needs more than 256MB to work, it will work on XP/Win7.
I have no knowlege of slowing down an Athlon successfully. I tried with an XP-M and a Geode, but the results were not as stable as I had hoped for. The competition isn't a slowdown star either, the P3 can be slowed down to 386 level, but not to something faster. So I would suspect that you will not be able to play much from the list of speed problems.
If you want to use that system as DOS machine, check out how the board works with himem and ems386. How much memory can you free up?
Regarding the AWE64: People who were around longer already had interest in older DOS games and usually had aquired one of the better DOS sound solutions. With the turn towards PCI and Win98, they didn't all give up their favorite DOS games. We learned quickly that most early PCI DOS Sound emulations sounded awful. So it was an obvious choice to keep the best ISA sound card around while possible. Or put it back in after hearing the SB PCI card. I wouldn't worry about an AWE64 in a build with ISA slot.
Retro PC warning: The things you own end up owning you.