I live in Uk too and have just built my first computer. I didnt buy a gfx card cos i didnt have enough money and so im still using my ATI x700. My build cost me about £650 which i think is extremely good for wat i bought. If i bought a new gfx card and monitor then it would have come to around £1000 so you should defintly be able to get some better components.
I would recommend getting either the Opty 175 if you are going to be overclocking or the 4400+ which i have. And to answer your question about 'are dual cores really gonna help that much in games?' then i would say YES! in November UT2007 will be released and this is the first game (i think) to use dual core cpu's so if you have a single core cpu then good luck on trying to play that game. I would also recommend getting the Hiper R type 580 W psu so when u get SLi it will definetly be able to handle it.
I recommend buying from www.ebuyer.com www.scan.co.uk
These are the best 3 places to buy from.
My specs are below