You could get buy with a 9800GT on that rig. They require a minimum of 400W, but from what I've seen, the Dell power supplies are pretty stout and are capable of running high powered video cards.
From a review of the Dell XPS 410 at Anandtech
Some might be concerned that the 375W PSU may prove insufficient for driving a GX2. NVIDIA recommends 425-450W for the GX2, but depending on PSU quality you may be able to get by with less. Our power draw results indicate that there should be more than enough power available for a GX2, as previous testing has shown that a GX2 card doesn't consume substantially more power than a 7900 GTX due to the difference in clock speeds.