On the prebuilt side, if you wait for a deal on an XPS system they are actually well built and engineered. I forgot the site, was it extremetech? Were they opened up one and overclocked it. It was pretty much only held back by the slower memory built into it.
I think best bang for buck is either an 8800gt - Remember we're due for a shrink so we don't know yet whether the new g92s will xfire with the old ones. That or 9600gt SLI, you can get wicked deals on both for 300 or even as low as 220 on new egg with MIR. One thing concerns me with going dual 8800gt is the fact that the single slot design dumps heat into your chassi, so I would feel that with the 9800gts out the 8800gts g92*2 would make a better bet.
Lastly, look at those combo deals on newegg! You can often save over $100 by getting parts that may not be exactly what you want, but fit your purpose. Like my aforementioned Phenom deal.
P.S Don't be afraid to go all AMD if the price is right. They have some exellent utilities for overclocking all AMD systems, makes it easy. Lastly if you're not afraid to overclock but you've never done it before... Try your hand on the Phenon 9750 black edition - good price and with powerplay you save $$$ when not gaming.
Edit: All I've seen on larger HDD is that in general their read and write performances increase NOT decrease with density of the disk.
Tease.
