After some research I learned that the last processor that can work on that kind of motherboard is Intel Pentium 4, 3.4 GHz ??? I am not sure can somebody help me :?
As with pretty much all OEM computers the processor upgrade list is very limited and this seems to be the case with your computer. According to Dell's website the only CPU's that will work with that motherboard is:
"Intel® Pentium® 4 and Celeron®; design provides for future Dell-supported upgrades. A slower compatibility speed can be set through system setup."