Well, i think your stuck in what i call limbo. Your not up to date on tech but not to old either. 1 thing i have noticed is your CPU, the AMD Athlon 64 X2 5600+ is a shocking cpu & i find single core cpu's better performing.
I would find the type of CPU you would like, Phenom II x4's are decent CPU's as i currently run from one myself. Then get the motherboard to suit. If your not into all this overclocking & benchmark bragging than any motherboard that has the option for upgrades of better cpu's & has USB3 & 2, sata II/III & 4 DDR3 slots for more expansion room should work well. HDD well, if you still have plenty of room then no point buying bigger. The GPU, if it handles the games you play well then why upgrade now when in the future the one's that cost an arm & a leg will get cheaper.
I woudnt run any system from a PSU unless it was a 450 80Pluss or better as these are reliable & do give you 450w & unless these cheaper versions that say 600w but barely push 400w.
If your not to fussed about buying new then you can get cheap ddr3 from ebay. Decent cpu's from ebay also but if your buying something that's the brains & heart of your system like CPU & Motherboard i would buy new. So like what's been suggested to you, i would also look at ebuyer.com. Note down what you like & search around for the best price. I got a HD7870 GPU for £130 & then found one elsewhere for a flat £100 so it pays to look around.
