It's very hard to say if i'm to be honest. Dell systems are not forthcoming on the internals used. From what I can gather, that system uses either a Pentium 4 520 or 540, which means it uses the later generation LGA775 Socket rather than 478. The problem, largely to do with the chipset, is that you are limited to 90nm Pentium 4 and Celeron D CPUs, not the later generation P4 Ds. So the best CPU you could probably get is a P4 670, but you would need a new power supply as well, like a 300 watt+. Basically, any 90nm LGA775 based P4 can be used (Prescott series).
Your options are very limited. You could buy an Atom based nano-ITX motherboard with CPU on board for the same performance, lower power, and cheaper, or even one of AMD's new fusion based systems that might fit inside the same SFF case.