Ok, that was stupid on my part . Updated the spec to include 2 front intake fans and the Maximus Formula. This now puts you about £40 overbudget, but it would be worth it IMO considering what you're getting. If you really can't afford much over £600, then you can always opt for a cheaper MB, something like:

Abit IX38 QuadGT - £117
Or
Asus P5Q-E Intel P45 - £100

The second one would only give you x8 on the second 4850, as opposed to x16. This would reduce the performance of the second 4850, but not by much.