get the Corsair 620W PSU, its better and its like a 700W performance i have one myself. for a videocard if 400 is not too much for you i reccomend 5870 if you are up for 600 get the 5970 and if your budget is a little tight get a 5850 or a 4800 series card. (i have a 5870 it runs everything and i love it) for a motherboard, i reccoment gigabyte and if they sell them in UK get a DFI board. i heard theyre very good, durable and built for gaming. there expensive and rare tho. id just get a gigabyte, they make good stuff (made my 5870). then CPU get a dualcore or a quadcore above 2.5ghz. AMD/Intel whatever you like best. (i have a pentium D 3.4ghz it rocks but i want to upgrade to a 3.6ghz p4-D) and make sure you get a big and wide case if you want to get good cooling in there. they can be expensive, but you can do what i did, i got a very old server case for 10 bucks from a computer shop and its pretty much perfect. well thats my advice
also for a cpu fan if your case can fir it, get the biggest hunk of copper you can find, dont be afraid to pay 100 bucks for it, its worth it, will keep your CPU at 20c like my zalman. (well i also use a soviet industrial grade thermal paste КПТ-8)
sorry i thought you were from UK for a sec. If you want something to last you at least 4 years, get the 5870 or a 5970 there frigging powerful and there are tons of new games coming out that use dx 11 this year so you wanna stay on top of that. and about AMD being inferior to intel AMD dosent give a crap about heat or energy efficiency, their CPUs are little monsters and cheaper than intel. you just need to get good cooling for them. as i said the biggest hunk of copper you can find. Also AMD phenom tricores are actually quadcores with one locked core, you can use some hack to unlock the 4th core (obviosly voiding warranty) but now you will have a fully functional quadcore CPU for the price of a tricore
and about the PSU the corsair 620W is very good but really if you can get a 750W. really if youre planning to get sli or crossfire youll need to have at least that much, id get an 800W if i were you.if you are planning to upgrade the comp over time, you never know what hardware is gonna come out so if you upgrade start with a good PSU the more wattage=the better for your system (wont get strained by lower wattage it wears out and damages your system)
dont hate people who rip you off,cheat and get away with it, learn from them