zehef,
What hardware have you already collected to assemble this system? Do you have any video cards at all, or are you still trying to decide which one to purchase?
My advice still stands: Use your Athlon 3000+, 512mb RAM, Win98SE, and FX 5900 or 5950 Ultra and use wrappers for GLIDE games. If you can't get an FX series card, then consider a GF4 Ti4200, Ti4400, or Ti4600. I think you'll be happier in the long run. You'll also be able to run more demanding games, should you choose to do so. This is also your cheapest option. You could always find 3dfx stuff later if the wrappers fail to meet your expectations.
I've heard that Voodoo 2 cards can run much hotter than normal if you use faster processors. If you use Voodoo 2 cards with your Athlon XP 3000+, then you may want to attach heatsinks and point a fan at them.
Personally, I've got two Athlon 1400 systems with Voodoo 2 cards in SLI mode. I have attached small heatsinks to the Voodoo 2 cards and rely on airflow within the case to cool them. GF4 Ti4600 cards are the primary graphics adapters. This setup works just fine and I have no problems using the FastVoodoo2 drivers. As such, I assume your setup will be just fine as well.
Before you buy a slower CPU, why don't you try underclocking the Athlon XP 3000+ and see how low it can go? Based on what you said earlier, it sounds like you have the 166mhz FSB version of the Athlon XP 3000+. I checked the manual for your KT3 ULTRA2 and it looks like it supports setting the FSB to 100mhz, so you may be able to underclock your CPU to 1.3ghz. In this configuration, it will run cooler and use less power too.