As an exert from the lowdown of your machine says:
Expansion is easy in this system with only one exception. The Dell E521 provides a full x16 PCI-Express slot for graphics expansion to a discrete solution, but the power supply lacks the PCI-Express graphics power connector for high-end graphics cards. Unless you upgrade the power supply (it is a standard ATX size and format)
http://www.ktechpc.com/delle521/e521.htm
Edit: hahahh.. sorry bout that, actually neither of those 9x series cards require additional power either, so it should be fine too. Silly me not noticing.
But you wanna keep that in mind if you ever look to upgrade your computer much more in the future ^^;