According to the CPU support list for your motherboard it will support the Core2 Quad models on the LGA775 platform. So your best bet would be to go with the best C2Q chip that you can afford. You may need a BIOS update to recognise the new chip and this can be obtained from the ASUS website. Best of luck!
Apologies, I thought the OP said he was after the best CPU he could upgrade to. Obviously it depends on your needs/usage and the rest of your system if you're looking for one that would suit your own needs.
Your board will support any Core 2 Quad up to a Q9650. If by any chance you have a MicroCenter near by you can get a Q9550 for $179 or $189 I cant remember which but I know its cheap and that will give you performance on par with a Phenom II 965.