If it's a 2008 Mac Mini, it's upgradeable in theory but not really all that far. It's only capable of taking 667MHz Core 2 Duos on an old socket (Socket M), so the max you'll get is 2.33GHz.
Also, your max RAM in the Mini will be 3GB that you can use - if you add 2x2GB to it, and go to About This Mac you'll see 4GB there but if you launch Activity Monitor and look at the memory tab you'll see 3.ish because the system can only address 4GB total due to a chipset limitation.
If I were you I'd sell both systems and get a souped-up current-gen Mini or maybe an iMac (which doesn't cost a fortune to get up to 8GB) if you don't already have an IPS display.
I do not understand what I do. For what I want to do, I do not do. But what I hate, I do.