Wow, your in luck I have the same PC. Sorry, but for X10 you need a graphics update. The only good card I can think of that the power supply will support is a standard-clocked Nvidia 8600GT. You can get one from Dell for the E521, and it should work. I was looking to upgrade myself, so I hope my research has helped.
(And yes, It will support Vista out-of-the-box.)
Speaking of Vista, I haven't encountered any bugs with a program made for Vista...yet. Backward-compatibility is a different story. It helps to run these programs in Admin mode.
The best things in life are edible.
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