I was digging through Google to see if your motherboard supported Core 2 Duo and came across your post on AV Forum:
Intel Core 2 Duo E7200
Asus P5K-E Wifi/AP Motherboard - where can I get this for £60?
3GB DDR2 (2 x DDR PC2-5300 & 2 x DDR PC2-5200) (Existing memory)
Corsair PSU (Still unsure what one exactly? Could you recommend one?)
1Tb Samsung SpinPoint hard drive
Arctic Cooling Freezer 7 Pro
New case - will any case do?
New graphics card - can you recommend one?
I don't know how much you want to spend, so here are a couple of budget-minded suggestions.
Case-Lian Li PC-7B Plus II. Build quality on Lian-Li cases is very good.
For the video card, the 4850 provides great bang for you buck. It would help to know what resolution your monitor is also.
Corsair PSU's are good, I have one in my dad's rig and it's been running 24/7 just fine for the last several months. This one's modular so cable management will be easier.