All is not lost! I have had EXCELLENT results overclocking the Riva128ZX chip! Download Powerstrip or a similar overclocking program and try it! BTW, the stock heatsink just won't do for serious overclocking-find the biggest self-adhesive heatsink/fan that will fit (it looks like you hav room for even a Pentium cooler there!), and put it on. If a Pentium cooler won't fit, use a 486 cooler. If you can't find one, ask the local junk computer dealer. Or use a blue orb. If you can't find one with adhesive, get frag tape from an overclockers site. Or Artic Silver Epoxy. You should be able to get about 50% better clock speed out of it with proper cooling. You'll be amazed at the results!
Because of the limits of PCI you would be better off overclocking your graphics chip or buying a motherboard with AGP slot.
Suicide is painless...........