I don't see a problem with what you are wanting to do. I would make sure that your motherboard will support the Q6600. Might need a bios upgrade. You mentioned the socket type, but not the Mfg or the model # of the motherboard.
For the memory, are you purchasing a 4 gig kit to replace what you currently have or you purchasing 2 gigs to add to what you already have? Reason I asked is due to limitations of memory slots, some people have to buy a whole new kit to replace what they currently have.
When you said you are on a tight budget, how much are you willing to spend?
Get q6600, more RAM and a good cooler for the CPU and you will have a great system. Look on ebay for the parts. I had q6600 and it is best quad for the money. A new GPU next year will keep your system going for a long time.
What is your problem?
Why do you think you need to upgrade?
As a rule, the graphics card is more important than the cpu for gaming.
To help clarify your options, run these two tests:
a) Run your games, but lower your resolution and eye candy.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
b) Limit your cpu, either by reducing the OC, or, in windows power management, limit the maximum cpu% to something like 70%.
If your FPS drops significantly, it is an indicator that your cpu is the limiting factor, and a cpu upgrade is in order.
It is possible that both tests are positive, indicating that you have a well balanced system, and both cpu and gpu need to be upgraded to get better gaming FPS.
Most games will only use two cores, perhaps three. Upgrading to a quad may not be as effective as you hope. You might do better to overclock your E5200 a bit. It is a free upgrade.
I also think that with only 2gb of ram, you may have a problem there.