LOL a 295 on a stock 350 watt Dell PSU I truely doubt it. The the GTX 295 at full load draws about 310 watts in itself so JP please do not give out such bad information without actually knowing we dont speculate too much when it comes to system specs we give the raw facts and will tell you what you need.
I'm sure you can run up to a 295 on the PSU with NP's
If you put a 295 on that PSU it would most likely not even power up or might even catastrophically fail pleas refrain from giving uninformed advice thanks. And besides the fact that the PSU doesn't have nearly enough power to run the card I doubt it has an 8 pin PCI express power cable eithier. I am not trying to be mean but please dont post if you have no clue what you are talking about.
The most you could safely run is something like a HD 4670 or even a 4850 might run on it I had a 9800 GT on a dell 350 watt PSU for a long time and it ran fine.
The XPS 420 came equipped with either a 375 watt or 425 watt psu, niether of which would be capable of powering a 295.
The best thing for the OP to do would be to tell us what video card he wants to upgrade to, and which of the 2 power supplies he has, he may not have to upgrade the psu at all.