It's more or less on par with a Intel Slot 1 rig.
It was never as refined as Slot 1 but that's to be expected as AMD abandoned it for Socket A relatively quickly.
I've just finished playing NFS 4 on my Athlon (non Tbird) 600Mhz with a 3dFX Banshee and plays fine.
It dual boots into dos and plays games just fine, but I'm not downclocking or anything.
You'll need a "goldfinger" to really unlock multipliers, there is an active thread going on at the moment
AMD K7 "goldfinger" cloning thread
Motherboards are more rare the CPU's so I'd say grab it. Double check what chipset the motherboard is using
The original AMD 750 Irongate based boards have buggy AGP (hence the 3dFX card in mine)
VIA KX133 have buggy so better off using a PCI card of some sort.