The CPU you have is not a bad one. To upgrade you could probably go to a Phenom II 980 as a max. It won't offer a massive performance improvement.http://www.tomshardware.co.uk/gaming-cpu-review-overclock,review-32395-5.html
The 820 isn't listed but the 810 is on the third from top tier on the AMD side. More detail the exact model number from the motherboard would be helpfull to confirm processor compatibility. You can often see it on the bios screen when the PC is first turned on.
Why do you want to upgrade?