I installed a GTX750Ti O/C 2GB DDR5 in one of these. I removed the bracket and cut in 1/2 top to bottom, then trimmed the plastic cover back about 3/4"" so the air could turn and go out the vents. No extra power cable required. Whatever card you get go for DDR5 frame rates are much better. I know this is an old thread but people still look at it.
CPU can be improved and these are very inexpensive now Core 2 Duo E6700 (not Pentium E6700) 2.66Ghz
Others are Q6700 @.2.66 Quad,X6800,QX6800 both @ 2.93Ghz QX6700 also2.66( X chips won't be unlocked in BIOS)
Be sure you have BIOS 2.4.0 Throttlestop software will unlock X chips
This system supports 8GB DDR2 @ 800Mhz so 64 bit OS is an option.