You have a 945 chipset on an OEM machine so probably not. The first chipsets to support C2Ds were the 975 and then 965. 945 did eventually do it, but not the first revision boards. I'm going to assume that as a dell you have one of the first rev boards, and they never released a bios or board upgrade to support C2D. You should be able to upgrade to one of the 8xx or 9xx dual core P4s.
Wouldn't worry about that 375W PSU btw. Its a tough unit that can handle most GPUs.
Thanks the hep..............