I wouldn't bother with the Chieftech power supply. You could just go on eBay or some other marketplace and find a clean used Seasonic 350W 80plus white/bronze unit like the 350ET, 350BT or 350ES (there are probably others too, just check what combination of cables they have as these were made with either lots of older or lots of SATA connectors). I have used dozens of these and they never have issues running systems from this time period. They're usually rated at either 20a or 25a on the +5v rail with 103w on 3.3 and 5v combined. You don't need a lot of juice on the 5v rail for this system.
Also the GeForce 3 should be plenty to run anything that the 600Mhz Katmai will be fast enough for. Remember, when that CPU was released the fastest video cards on the market were the Voodoo3 and TNT2 Ultra, with the original GeForce coming shortly after. The GF3 will be significantly faster than either and it comes from a time when the fastest PCs may have been running P3 Tualatin, Athlon or even P4 CPUs well over 1GHz.
If you want to crank up the resolution of games from the late 90s to 2000 or 2001, you could get a beefier GPU, but I think you should just run what you have and then see what improvements are needed if any. Realistically, demanding games made beyond the year 2000 will likely not run that great with the 600mhz Katmai so your GPU won't be the limiting factor there.
Now would be a good time to make a list of games and programs you intend to run so the build can tweaked for your actual needs. That is the most efficient way to do this without spending money unnecessarily or being disappointed by unplayable performance.
Now for some blitting from the back buffer.