Dell deals have been a bit thin on the ground this week and I see Rayman has kindly posted some for us while I was indisposed yesterday.
Anyway here is a whisper quiet 5150 that ticks all the right boxes, there is another with the same spec for £499 (D07515T) but this has another 512meg of memory for £20 extra.
Intel® Pentium® D 820 Dual Core Processor (2.80GHz, 800MHz, 2x1MB cache)
Genuine Windows® XP Home Edition, SP2 (incl Symantec PC Restore Recovery)
1024MB Dual Channel DDR2 533MHz [2x512] Memory
160GB (7200rpm) Serial ATA Hard Drive with 8MB DataBurst cache
16x max. DVD+/-RW Drive
128MB ATI Radeon® X300 SE HyperMemory graphics card
Dell 19" Value Flat Panel (E196FP)
Integrated Audio with Dolby Digital 7.1 capability
Free - Dell Colour Printer 725
You may want to consider a cpu upgrade to Intel® Pentium® D 930 Dual Core Processor (3.00GHz, 800MHz, 2x2MB cache) for £35.25
As usual reduce the warranty to 1yr and of course buy Via Quidco for further discount.