If you're not interested in XFire or SLI I'd get the ASUS A8N SLi Premium motherboard as it's a nice, stable proven board
The 256MB X1800XT is a bit of a bargain at the moment and it's still a very fast card. The X2 4400+ has the extra cache over the X2 4200+ which gives a fair overall boost in performance and a large boost for cache intensive utilities. You're likely to keep your CPU longer than your grapihcs card (on average) so I'd go for the faster chip.
The X2 4400+ will easily clock to X2 4800+ speeds - it might even make FX60 speeds if you're lucky.
Not wishing to confuse you any more but the Opteron 170 is also worth considering. It's slower than the X2 4400+ at stock (2GHz vs 2.2GHz) but it is an amazing overclocker - they're all reaching 2.6GHz on stock volts or with a slight voltage bump. This makes them an FX60 chip - very, very fast!
This would be the chip I'd go for
EDIT: Just read Xionic's post above. I didn't realise that the 512MB X1800XTs had dropped so much - that's definitely the better option.
I'd go for something like:
GX-048-PC PowerColor ATI Radeon X1800 XT 512MB GDDR3 AVIVO TV-Out/Dual DVI (PCI-Express) - Retail (GX-048-PC)
£219.95 £219.95
MB-111-AS Asus A8N-SLi Premium nForce4 SLi (Socket 939) PCI-Express Motherboard (MB-111-AS)
£104.95 £104.95
MY-007-GS G.Skill 2GB DDR ZX PC3200 (2x1GB) CAS2 Dual Channel Kit (F1-3200PHU2-2GBZX) (MY-007-GS)
£129.95 £129.95
CP-148-AM AMD Opteron UP 170 Dual Core San Diego 2.0GHz 2MB Cache (Socket 939) - Retail (CP-148-AM)
£269.95 £269.95
Subtotal £724.80
VAT £126.84
Total £851.64