Thanks for your reply. Have been looking at CPU's, etc, and maybe I don't need anything like an i5 yet. How about the below improvements on my current system:
Upgrade CPU from E5200 2.5GHz to Intel E6700 3.20 GHz
Upgrade Ram from 4GB to 8GB
Upgrade main system hard drive to 64GB SSD
Add Wifi card
Are you currently having slowdowns? I wouldn't upgrade unless you really need more speed. The goodness of PC is that you can overclock, that CPU should go to around 3GHz without issues. What RAM does your system use? If it's DDR2, don't as it won't work in your possible future machine (future'proofing your purchase). I wouldn't grab SSD yet, they are coming down all the time.
If it does what you want it to do, just let it be. Save the money towards bigger update in the future (possibly Sandy Bridge in early 2011) and then pull the trigger. Of course adding 8GB RAM and switching the CPU to quad core would make it a decent machine for years but for almost the same money, you can pull the trigger on new mobo and a lot newer CPU