I've got the DS 😀 also limited to 600Mhz Katmai
Asus P2B-DS Build
Keeping in mind your Athlon64 Win98 PC I would:
SCSI HDD of course!
3dfx card, you already have awesome d3d box with the ti4600, make this your glide box
Vortex 2 would be real nice if you can find one for something different, otherwise cant go wrong with a Audigy 2 ZS. Anything else is ok as a budget option.
AWE64 for late dos gaming, I'm sure your earlier PC's are more suited for anything that wants true OPL.
OS I would triple boot
Win2k, again just to be different from your other builds, and it actually makes a real good Win9x platform
Win9x for the few games that don't like Win2k, and lets you drop back to Dos
good thing is Win98 supports the on board scsi right out the box, even Win98 dos can use the HDD's without any messing round with drivers
This my most used PC for retro gaming. It just plays about 95% of my retro collection, (the Voodoo 2 SLI is why I prefer this over my Slot 1 1Ghz 4600ti based system)
and really yours is more practical, I know I don't need 2 CPU's..but damn they are sexy 😉