FYI, the (quad core) Q6600 runs on a 1066 MHz FSB, whereas the (dual core) E8400 runs at 1333 MHz. I don't think the mobo allows you to overclock.
I'm running a Radeon HD 6450 in a 755 SFF; I didn't want it for gaming, just needed something decent for a Linux install. The card does have to be low profile and needs the low profile bracket. Also, beware that the graphics slot in the SFF is immediately next to the hard disk mounting, so you cannot use a double-width graphics card.
Also, in case you didn't know, the spec of the 755 SFF is that it can take up to 8 GB RAM (4x 2 GB), but actually it can take 16 GB (4x 4 GB); I have one with a Q6600 and 13 GB (3x 4 GB + 1x 1 GB, awaiting the 4th 4 GB 'cos they ain't cheap).