What are the other specs of your power supply?
How many amps on the 12v rails do you have?
Since most Dell PSUs are a bit underrated, if you're lucky, your PSU might actually be around a 400-450watt PSU.
In the best case scenario, you can run a good ATI 5770 (equal to the 4870 or GTX260).
If not, you can probably run a 5750. (performance equal to a 4850 or GTS250/9800GTX)
In the worst case scenario, you'll have to go for a 5670 (performance between a 3870/9600GT and a 4830/9800GT)
btw, a GTX285 is overpriced and power hungry. In terms of price-for-performance and power consumption, ATI's current 5x00 cards are MUCH better than Nvidia's old GTX200 cards and slightly better than Nvidia's new GTX400 cards.