No need for a dual system, it will actually perform worse than a single CPU system because games don't use dual CPUs and the memory they need is higher latency.
You're better off getting a modest Athlon 64 system such as:
Athlon 64 3400+
1GB PC3200 DDR RAM (2x512MB, the best you can afford)
An NForce 4 chipset mobo when they come out in a few months or maybe an NForce 3 250. (try Asus or Abit)
ATI Radeon 9800 Pro, which is the best value for money right now.
Western Digital Caviar SE 200GB HD (IDE version, 8MB cache)
Antec 450-550W PSU
Audigy 2 ZS
That little lot would cost you around £600-700 if you bought it right now.
www.overclockers.co.uk and
www.scan.co.uk are good suppliers, but avoid using Scan for anything like mobos or graphics cards.
You could also swap the Radeon for an NVidia 6800GT which are also very nice but cost about £150-£200 more.