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the GTX 480 is a dinosaur like your 470, I wouldn't consider it very much of an upgrade. A GTX 690 is out of your budget and a GTX 680 would take all of your budget, even a GTX 670 is $400. The best GPU value right now is probably the AMD HD 7870 for $280 or the 7850 for $220.
a i5 (non OCing) and a MB is going to be about $260+ if your old rig is running DDR 2 ram you need a ram upgrade to DDR3 also.