Hi Simon you budget is a bit tight for what you require, but here are some recommendations.
MB-Try settle with a H55 board you can find a decent one for about $200, prefrably Gigabytes as they are more reliable then others.
CPU-Try get a core i5 750 quad core cpu this should definiatley do the trick is say. $250
Memory-Try get Kingston Value Ram you dont realy need asnything of low latency just get a single 2GB DDR3-1333MHZ and then get 2 mosules (4GB). $75 X 2 = $150
Graphics-Here i woul recommend all you left over cash as much as possible, dedicate all that into the card for a decent card for your requirments i would look at spending $190 maybe look at a GTS 250 if they are still available or try to push it on a ATI 5770 as it is a very decent card.
Total on system $800 +/-
Not sure on pricing as im from SA i had to do manual conversions =)