Your 5770 is already a very capable card.
If you make an upgrade, you need to jump several performance tiers, or you will be disappointed. Something like a 6970 or GTX570.
Also, consider that your cpu may be a limiting factor.
To help clarify your options, run these two tests:
1) 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.
2) 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.
Normally for gaming, the gpu upgrade would be most likely to produce more fps.
Is your motherboard and psu able to handle crossfire 5770?
However, in this case, the Q6600, which was a fine chip a few years ago, is now outdated.
I would consider a change to a sandy bridge based cpu. Even the 2100 dual core at $125 should give you better results in most games.
Better yet, the 2500K at $220 is the current gold standard for gamers.
There are some well priced bundles around. Check microcenter. Look for a P67 or Z68 based motherboard that has good crossfire capability.
You could initially use your 5770, and other parts to see how you do. Sell the Q6600 and ram to reduce your costs on ebay. The graphics upgrade could be as simple as adding a 5770.
I think I would hold off untill the end of the year for buying a great single graphics card. The next gen graphics cards will start to appear then.
At that time, current cards may drop in price, but you probably will not want to buy them when the new cards are better and cheaper.
A cpu upgrade to sandy bridge will last longer. The 2500K will drive any current graphics configuration for a long time. The P67 and Z68 chipsets are supposed to be compatible with the 22nm sandy bridge follow on cpu's due out early next year. Only a bios update will be required.
It looks to me like there is only one pci-e slot. That would preclude 5770 crossfire as a viable upgrade.
If you are near a microcenter, check them out. In the past, they have had great prices for walk in customers on cpu's.
If you, like most of us, no longer have a need for any expansion cards, look at micro-atx motherboards. They only have 4 slots, but are cheaper.
In general, a M-ATX motherboard will cause you to plan on using a single good graphics card vs. cf/sli.
Some, like the ASUS P8P67-M PRO can do sli, but the two graphics cards would be mounted together, causing heat problems. http://www.newegg.com/Product/Product.aspx?Item=N82E168...