http://www.asus.com/us/Motherboards/P6T/HelpDesk_CPU/
The link above is to your support CPU's. As they are all pre-Sandy Bridge, I would not recommend a CPU upgrade unless you are prepared to get a new motherboard so you can go Haswell.
While there are good reasons to have lots of RAM, gaming is not one of them above a certain point. From the article below, across multiple games, there was little difference in performance between 4, and 8 GB of RAM on gaming benchmarks.
http://techbuyersguru.com/RAMgaming.php
About the only upgrade you can make would be to swap the hard drive for an SSD if you have not done already. Anything else at this point will mandate an entirely new computer with a new MB, etc. The other option is a new GPU.
Your CPU is not bad though. Here is how it compares to a new Haswell Core i7:
http://cpuboss.com/cpus/Intel-Core-i7-920-vs-Intel-Core-i7-4770K