1st upgrade - CPU cooler and a good overclock. When you upgrade your CPU later you can continue to use this CPU cooler so make it a good one, that fits in your case of course
2nd Upgrade - Good case fans, assuming you haven't installed case fans in all the available positions, for better air flow. May allow you to raise your OC a little and OC your 5770
3rd Upgrade -1tb (or more) 7200RPM Hard Drive. While it won't improve Game performance directly it will cut down on load times and boot times. By then you may be finding your 320 is running out of space if you install alot of games
4th Win 7, or 8 if it takes you that long
. Time to be gaming in DX10/11 rather than 9.
5th well by the time you get around to this new stuff will be out so if you need to upgrade your CPU or GPU will depend on the games you play and what's out.