Also watch out for the dell cases, becuase they are non standard they are generally not designed for a double height card. You should probably still be able to fit a double height card but it would require cutting a hole in the case or the card bracket.
It's the double height cards the feed out through the back of the case that will be the most problem. a double height heat sink that doesn't feed out of the back of the case should be ok.
Most 7600GS & 7900GS are single height and have fairly low power consumption (the factory overclocked cards have higher power useage.)
checking on ebay for cheaper cards is a very good idea.