That's cutting it awful close. I think it will work but what about the Amperage. if you look at the label on the side or underneath your PSU it should say the amperage for each individual rail. Look for the amperage under the +12v rail. You may have 2 different +12v rails or just 1 +12v. Either way for this card you want about 16-20A. This is especailly important in systems like this dell where the cpu and videocard both share the same rail.
My experience with dells was always that the PSU's in them were very conservative in their ratings. My old dell 8300 has a 250w psu in it and it has a 3.0 ghz p4, a gig (4 sticks) of ram, 2 HDD, 2 optical drives, and for a while had a x800xt AIW with a sound card and a tv tuner with no problems ever, so you should be fine i would venture to say