What I would personally fit into those PCI slots is a S3 Virge 4MB coupled with a Voodoo 1.
You'll also be needing a 14" monitor tops, because the 640x480 gaming res would look hideous on anything larger.
Oh... and don't forget about the soundcard: Aureal Vortex 8820 would also be a great choice.
I use an s3 virge myself myself but the thing to remember about those cards if you want to get one is the diamond and creative have issues with older games but the herculese and HIS brands dont. Heres a good reference