I followed the instructions in a post from EvilOnigiri about how to unlock the shader controls in RivaTuner, but it didn't have any affect. Is there some other way to unlock these controls in XP?
You're graphics card doesn't have separate shaders, so you can't enable their overclocking.
Ge force 8xxx series can set separate shader clock, 7xxx series can not.
GPUs are dirt cheap right now, a 8xxx series or a 9xxx series or ATIs 3xxx series will do wonders over your 7800GT. Not much more than $100, tho Id go for something closer to 150 range for excellent performance, if you want to upgrade, and get into DX10, not that DX10 is that great either