Your processor is most certainly going to be the limiting factor, not graphics...I have a 512 ATI (which is likely not as good as anything geforce) and graphic settings are completely unrelated to performance...as more objects (basically, ships and SC) enter the game, things can get slower regardless of my settings...
If you were to improve your computer for the sole sake of making it better for sins play (a noble, worthy cause indeed 
), I'd go for improving the processor...just remember, this game is not multi-threaded, so the individual bus speed and processing speed of each processor is what is most important....a 1.8 quad-core is not as good as a 2.0 dual-core when it comes to playing sins....
If you are having performance problems and don't want to upgrade your hardware, there are mods out there that improve game performance by reducing the size of files needed and preventing weapon effects lasting forever and random quirks like that...I have a friend that uses a quad-core (does sound and video stuff) which isn't great on the individual processor level....he says that the mod he uses improved the performance by a fair amount....
These mods don't change the game mechanics in any way...they simply improve the efficiency of the game and have very little affect on the graphics quality...
