E4500 is a massive upgrade mate O_o...
I'd go with this upgrade path, E4600 + 8800GT. Factual sorces have stated that it runs faster than a GTS and is cheaper, however its stock cooling system is dreadful, I reckon something like a VF1000 would do justice to this though.
If you are on a tight budget, go for a VX450W PSU, even though its rated at "450W", this PSU is far more powerful than that, it will support your upgraded setup very nicely (and quietly as well).
AMD Radeon X1950~HD4000 Support Clubhouse
“Mac: "Woah, what are you doing?" Mac says as he levitates in the air.
PC: "Playing Half-Life 2, this is my gravity gun."
Mac: "Well I can sort of do that."
PC: "Technically Mac, you have to become me first."” -From_Nowhere