PC Upgrade Assistance (UK)

[Kipper]

Alright, lets get the funny part out of the way by my telling you what my current PC spec is (basically).


MB: Gigabyte GA-MA78GM-US2H PCI-Express DDR2 Motherboard
Mem: Kingston HyperX 4GB (2 X 2GB) DDR2 8500C5 1066MHz Dual Channel
CPU: AMD Athlon 64 X2 Dual Core 7750+ Black Edition 2.70GHz
GPU: Nvidia GeForce 9500 GT 1gb
PSU: 550w
Mon#1: 24" 1920x1080
Mon#2: 19" 1280x1024
OS: Windows 7 Professional

I'm looking to do a decent upgrade without having to rebuild the entire thing; and keeping to a sensible budget (bang for buck, not all out spending).

I know that GPU is a desperate need - thinking of going for a GTX570 or something, they are expensive enough and the 600's are just crazy money.

As for the CPU, is it a noticable step up from a 2 core to a 4 core? My board can take an AM3 chip (I think) but the current processor is a top end AM2+.

Any suggestions? (I'm UK based for pricing).