CPU or GPU, which to change?

[McLeod]

My current system is as follows,

E8200 @ 3.0 ghz on MSI Neo platinum
4Gb Corsair Dominator
Gainward GTX260 Golden Sample OC'd
Antec True Power Trio 550w
Benq 1920 x 1080 Tn panel

This plays most games at high/highest settings fairly comfortably eg: Crisis 2, WoT, BFBC2, Shift 2.

Now I'm going to have a budget of around £400 to change bits in order to incvrease performance for upcoming games like BF3, SWTOR etc etc.

Would it be better to go for i5 2500k with board and memory or change GPU to a GTX570 or similar and get new psu too ?

What what be most effective in short term, long term I will be changing the rest anyway.

 

[McLeod]

cool seems reasonable, been looking at the Coirsair Vengeance Blue 1600Mhz 8gb set for about £50, suitable isn't it, or should I go for 1800's ?