if your looking for gaming performance, i would keep the GT since it has more pipes/faster bus/more ram. sacrifice the cas 2 extreme ram or the SCSI HD. those 2 will prolly give u the least noticable improvment if your talking about gfx.
I agree with jolancer. Get a Western Digital Raptor and save yourself 200 bucks on the hard drive. If you want to go even cheaper, just get a Western Digital 160 GB 7200 IDE drive.
yeah get urself a 10,000 RPM western digital raptor (same size as te one u listed above).. ud save urself money... and while gaming you wouldnt really notice that much of a difference.. actually u wudnt notice a difference AT ALL lol