DO NOT UPGRADE THE PROCESSOR.
I've upgraded the processor in my old Dell (Optiplex 320), it had a Socket 775 Intel Pentium 4 (3.0 GHz), when I bought a Core 2 Duo, the BIOS freaked out and didn't know what to do, and bricked the entire machine.
It wasn't just my PC either, it happened in my Mom's PC (same one as mine, but it worked afterwards).
You'll need a new motherboard if you want a new processor/CPU.