if you feel confident enough it might actually be cheaper to keep your current pc case but replace everything in it.
since you would only have to buy a motherboard, cpu, ram and a vid card you could be done in under 400 pounds, or even below 300 if you manage to find good parts 2nd hand, plugging a few wire's with the manual from the new motherboard is easy, assembling the rig take's only slightly more skill than that.
i recomend an am2+ socket motherboard or budget allowing am3, AMD sockets are notorious for their future compatible upgradeability (i used 3 cpu's on an am2+ mobo over a span of 4 years before i build myself an entirely new system because my motherboard died.
(i use an i7 now... but regretting it already because i know that for my next system i'll have to buy a new motherboard and cpu, instead of just a new cpu)
edit: @hielor, my bad (shouldn't post while under influence of alcohol)