You should look on ebay if you're considering a dell.
I'm a fan of the optiplex line, and have had tons of optiplex systems ( literally ). I just bought a gx520 for a girl I work with for $40+25 shipping on ebay 3.2p4,320gb hdd, 512 ram, xp pro.
If you're intending to do gaming though, you should probably go for the mini-tower so that you can add a full sized video card.
The 320, 330, 745, 755, gx620, 740, 760, and 960 should all have PCI Express slots. As far as a video card for gaming, I'd recommend an ati 4670 because it doesn't need additional power and it's still a decent video card.