its not your PSU, its the crap card and ancient Pentium D processor. The DELL XPS comes with the 375w PSU which is the best peace of hardware you currently have. It powers the ATI 4850.
I would suggest stepping up to an Nvidia 9600GT, GT240 DDR5 or ATI 5670 ver 2.0. Anything else and it just would not be worth it with a Pentium D. 2GB of system RAM would be nice if not already installed. 3GB or more if you upgraded to Vista or Win 7.