In your position I'd go another way:
Acer Aspire X1200-U1510A Refurbished AMD Desktop PC $290 dual core AMD Athlon X2 5000+ 2.6GHz CPU 4GB RAM 320GB HDD DVD burner and Geforce 8200 integrated video.
(there is also a $280 X2 4400+ 2GB RAM model of the Acer Aspire X1200)
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.
Passmark G3D ratings:
GeForce 8200M 74 Radeon HD 3450 144
Radeon HD 4350 231
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.