Thanks for the reply, Char!
Motherboard is an Asus P5K-E/WiFi-AP. I think I bought the thing because of the chance I could upgrade to a Quad if I ever wanted to, and looking at the CPU Support list, it does support both older and newer versions of the Quad Core. Also, I might go for a Q8400, which is significantly cheaper and can hold its own against the Q9400, if hardware reviews are anything to go by.
The graphics card is another thing I'd wanted to upgrade, but all in good time. It's an 8800GT 512mb. It's getting a hard time when playing the heavier games, but in my opinion, getting more processor power is a priority right now.