In my house I have an LG monitor and my wife has the Acer quoted above. It uis up to you which you go with, the LG is a bit smaller for the same kind of price but is technically a better brand quality - although ironically, at home my LG monitor has got a dead spot on it right in the middle of the screen now while the Acre monitor is still perfectly fine.
I just saw that you are not including the monitor in your budget, that is good news, so scrap the CPU and motherboard I mentioned as you can get better. So, without monitor:
This build does not allow for XFire or USB3.0 and Sata 6gb/s, but allows spare cash for a CPU cooler for OCing and posibly a more powerful Powersupply.
The best P55 board that will give you USB 3.0 and Sata 6gb/s and XFire is:
Asus P7P55D - E Pro - £163.97
This will break your budget at: £539.50
If Crossfire matters to you but USB3.0 and Sata 6gb/s does not, you can likely buy a board in between. (For me personally the USB3 stuff was of no interest, it is something that will matter when I get a new system in 3+ years).
Depending on games you play, you may find the 5770 starts to struggle at max settings on some games, but if you have an XFire board you can always put a second one in, or if not buy a 5850 in the future, but other than that the computer is good to go!
Note that I bought the RAM from www.ebuyer.com this is because, while I love scan (I have used them for years) the one thing they lack is RAM variety - G Skill is brilliant RAM and tends to have lower latency than the Corsair on offer at scan, so I always buy the RAM elsewhere.
I really wouldn't go with RAM at scan. I buy everything at scan for my PCs except RAM - at Ebuyer you can get better RAM cheaper even with the £2 delivery. G Skill Ripjaw is great RAM, and you will see it recommended often on this site
I5 is a good price at scan, you can check ebuyer and overclockers.co.uk but you will likely find scan is as good a price as you can get it.
F3 is reliable and very good - better than WD. It is no coincidence that almost every single build on this forum recommend the F3 - we are not on some commision, it just really is that good