With regards to the motherboard:
scan.co.uk/Products/Asus-M4A79XTD-EVO-AMD-790X-AM3-PCI-E-20(x16)-DDR3-1333-1600-1800-SATA-3Gb-s-ATX
That offers two x16 pci-e slots for all the crossfiring action you'll ever want (and probably won't need
imho )
And I'm always one for modular PSUs:
scan.co.uk/Products/520W-Corsair-HX-Series-Modular-PSU-ATX-EPS12V-whisper-quiet-5-year-warranty
Helps keep it nice and tidy, helps airflow too.
One thing though, I would not get the 4890 at this moment in time because you can have a 4870 for under £100 now and there isn't a huge performance difference between the two. You can even get two 4850s and have them in crossfire for £150 which would destroy the 4890 in terms of performance. Oh and DX11 cards are just round the corner. Just a thought
Otherwise the build looks awesome and I'd be happy to go ahead with it.
edit: oh and in case you were wondering, the 520w psu will handle crossfire with room to spare.
edit2: ahhh cooler!
scan.co.uk/Products/Arctic-Cooling-Freezer-7-Pro-v2-S1366-A1156-S775-AM3-AM2plus-AM2-939-Up-to-130-Watts!
Cheap and cheerful and gets the job done!