Seeing as it is a 7900 gs and it is a pentium D then it has seen quite a bit of service life. I recommend good quality and good price PSU units to replace OEM ones after a year or two to keep avoid any issues. Although Dell PSU's are quite good actually, aftermarket ones are better.
Here's two i love to use when replacing OEM psu's...
Antec EA 380D
(woops out of stock at newegg!)
Corsair CX 430 V2http://www.newegg.com/Product/Product.aspx?Item=N82E16817139026&Tpk=cx430v2