Go to scan, ebuyer, etc. to compare prices I'll just use OCUK for ease.
First thing I'd do is add in an SSD. You don't need an Intel 510. I'm looking at the Intel 320 at the moment and it seems to get some pretty good reviews for price and speeds/performance.
2 choices that pop out for me are the Intel 320 and OCZ Agility 3. Both are cost effective SSD upgrades. You won't get blazingly fast performance but from a traditional HDD to an SSD you'll feel the difference. And not to mention that you'd pay significantly more for better performance. Intel uses it's own internal controller and OCZ uses Sandforce controllers. I don't know all about the intricacies of each but I'm told they're both pretty good choices. Also, here's a tip, when looking at SSD's don't look at maximum read and write speeds, look at 4k and 8k random read and write speeds. You're more likely to do that than sequentially reading large swathes of data.
Second upgrade should probably be a video card. The 4870x2 is an energy hog and although still holds it's own you could get something like a 5850 and cut energy usage significantly, probably around half of what your current GPU pulls out of the wall right now.
I'd say a 6870 is probably the best option for a GPU upgrade at this moment in time. After that I wouldn't touch anything else and just get a new rig when your current one starts to show it's age.
Either one of these look pretty good: