Genesis (beloved) u can perhaps buy a PCI videocard. I don't recommend it, since Regular PCI slot is alot slower then the regular AGP port. I recommend getting a new motherboard (supposing you built your own) If not, you will indeed need to buy a new system.
I just borrowed an account from a friend to play for about 30min. It's looking good! I get great fps also. 640x480 (my CS1.6/Halo/ and what not fav. resolution) full settings and about 90-100 steady framerate. I hope i can manage it.