I think pretty much any PCI card "should" work in there since the 64bit PCI is backwards compatible ...
I am not aware of any 64bit PCI vid cards though ... so you would only wind up with a regular pci card ...
newegg has a pretty good selection of PCI cards though they are a bit pricy
gf2mx 200 PCI for $37,
Radeon 7000 OEM PCI for $41,
GF2MX 400 PCI $49
XFX Geforce4 MX440 SE 64MB SDR PCI $57
Prolink MX 440 SE Video Card 64MB DDR $67
POWERCOLOR ATI RADEON 7500LE 64MB DDR $69
VGA PNY Verto GEFORCE FX 128MB DDR PCI 8X 5200 $107
Newegg Just go there and go to shop by category, video cards, and then put PCI as a keyword
IMO the best buy of the bunch is prolly the the GF4 MX 440 DDR ... though its like the speed of a GF2 GTS if i remember correctly ...
It should be playable with most modern games
the 600SC is a pretty solid little server ... and the integrated vid (though its pure crap for games) is quite good for 2D. I run mine as a linux NAT, SQL, Apache, Eggdrop, Samba, Bitchx server and chatstation .. It has never let me down.
edit .. fixed link