hmm, well you're pretty limited for upgrades. you only have pci slots which is an old interface, you have a slim tower so you can only use low-profile cards, and being a slim tower it must have a weak small form factor psu, which knowing dell, it's probably proprietary (only fits dells) and probably has proprietary "dell" power connections.
on top of that, you only have a single-core cpu which is going to limit pretty much any newer card.
i really think you should just save that cash, save up some more and start the process of rebuilding a better rig.
if you insist on going through with getting a gfx card, this is probably your best bet in your situation even though it's not really worth nearly that much.
Newegg.com - JATON VIDEO-498PCI-DLP GeForce 9500 GT 1GB 128-bit DDR2 PCI Low Profile Ready Video Card