1/2 grand computa machine!

i have around £500-£600 to upgrade my current system what should i spend it on? ive started loads of threads about buying a computer but i think i can get better quality and more better stuff that i want rather than having to skimp on components. so i was thinking about new technology, conroe in perticular,
i need a good upgrade so i can
a) perform a proper ugrade (i havent done one or built a computer before although ive done loads of research)

b) so i can get a decent system to play new games fear on lie high or med settings at a decent fps

c) so i can overclock and make my computer look really impressive (thinking of conroe here) - haha
QUESTOIONS
1)if i buy now will i be one of those people who buys the system and 1 year later my £150 odd mobo will be incompatible with like a newer socket design? or ddr3 memory etc....

2)do i also need to invest in a northbridge cooler/ liquid cooling (considering that the fsb would be at like 1.6ghz 400mhz qdr) to achive 3ghz on a the cheapest conroe. what would any of you suggest i buy to achive this huge fsb overclock?

3) is the quad core gigabyte mobo actully kentsfield compatible?i aws thinking about getting that one

thanks any suggestionsa of how to upgrade my current system (retail phillips mt2400 its in the sig)would be much apprichiated thanks

thanks i think ill get:
xfx 7900gt
£240
thermaltake bigwater 745
£70
2gb corsair ddr2 800 cas 4 xms2 ram (epp)
£200
also i was thinking about throwing in a 750w thermaltake toughpower psu for like £70! but should i get this or just get the thermaltake tr2 550w psu for £45. is there a big difference between the high end psu and the budget psu (i would like the computer to be quiet because at present its loud). there is only a £30 difference heres the link:
http://www.techfever.co.uk/products.asp?partno=8246
http://www.techfever.co.uk/products.asp?recnumber=2889