If you are planning on upgrading your entire system but only want to do it one piece at a time as the money becomes available then I would start with the video card as suggested. Either card, the ATI 6960 or the 580 with the nvidia chip. The choice usually comes down to brand preference. I named the two newest and best to date only because if you're going to upgrade might as well look at the best first - if price is within budget. Unless you're willing to wait a little longer - when the 6990 comes out. So far it's supposed to be untouchable.
Next would be the CPU. I would go with the AMD Phenom II X4 955 Black Edition. It's a great overclocker when you want to get to that. And some games are already using the extra cores for play so it is nice to have when you run across that game you like but requires a little more processing power than you currently have. You could go with the 6 core if you really want to try and future proof it but I don't want to try to oversell you on this one upgrade. But, if you have the money to spend I would get it.
At this point I'd like to know what PSU you have. Is it up to the task? I see you said 750 watt but which one? Some are more capable than others. If you have a Thermaltake, Seasonic or Corsair, (just to name a few) you might be ok cause the new 6990 only uses about 300 watts, (ALL by it self), at peak usage. However - if you have some off the wall brand you should really get something proved to hold up under demanding usages and is rated at >80% efficiency.
Next would be the Hard Drive. It looks like it's time for some SSD's to go with those 6 GB s-ata's. Get one and see how you like it then grab another one for a raid setup for some increased speed.
Then of course it's now time for that extra 4, 8, 12 or 16 GB's of memory. If you already have a set of 2 X 2GB then just grab another set of the same make and model and you should be good to go with 8 GB of memory. However - If your interested in really being able to increase your memory storage capacity then you can get the SAMSUNG M378B5273BH1-CF8 4GB SAMSUNG K4B2G0846B-HCF8 1.5v memory. Start with 1 or two 4 GB stick(s) and before you know it you will have 16GB's of memory. Mostly useless in today's games and programs - unless you're doing video conversion and/or rendering or running some Virtual desktops. But if your looking to upgrade your entire system that's the way to max it out.
Sorry if I went to far and suggested things you're not even thinking about. I just figured I give you a big picture and let you focus on what you like. And it is kinda fun
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And this is all beside the point if you're willing to make the switch to the new sandybridges coming out.