You should ok with power, just make sure the card has enough space since most 250 are dual slots. You can also get the new ATI 4770 and save yourself some cash, space, and power needs.
Always listen to Delluser1, he always gives well informed advice about all systems, especially Dells. Just make sure that the HD4770 was available when his advice was given, it matches up very well against the GTS250 512MB, but gets beaten by the GTS 250 1GB (depending on game/resolution/settings)
You are about to answer a thread that has been inactive for more than 6 months. If you still wish to proceed, please ensure that your posting is original and does not duplicate or overlap any prior responses to this thread.