I'd go for the more powerful computer. The integrated Geforce 8200 will handle very light gaming and handle all the rest of what you want very well.
And the next time you have a spare $20-30 you can go for the video card upgrade.
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.