If you are looking at a 2 - 3 year ROI, then there are some upgrades to the 775 platform that would be beneficial. Keep in mind that this money could be put toward a new build as well... Upgrading that old of a system sometimes is not worth it, but if you know you will be using it for a while it may make sense to upgrade.
That system with gaming for the next 2 - 3 years...
CPU - Q8300 or Q8400. Stepping up to the Q9000 series has you spending enough to go back to the "Upgrade vs Build New" debate.
Memory - Upgrade to 8GB (add 2x2gb).
GPU - 6850 will do you well and carry forward to a future build.
What resolution do you game at? What is your budget? The weakest link in your setup is your graphics card.
Tom's Best Graphics Cards for the Money suggests that the 6850 is somewhat better than the 5770. Unsurprising, since it's a newer card, but it is also a recommended buy at its price point. The GTX 460 1 GB is also a very good buy that might be a little less expensive.