Yeah that looks good
For your budget you may as well go for the i5 I think, for cheaper builds AMD chips are good value for money, but IMO why not go for the newer tech if you have the budget.
Any reason not to go for the Powercolor 5850? They're all the same basic card unless it specifically states overclocked or whatever so no reason to go for a more expensive one, unless you have something for/against a certain manufacturer. Plus you get Dirt 2 with the Powercolor one free! Don't know if you do with the Sapphire.
Also from the i5 spec:
Memory Controller: Dual channel DDR3 800/1066/1333 MHz
So 1600mhz is not needed really.
Not sure what the cooler you chose is like, maybe try and google for some reviews of it.
I think 550w PSU is enough, and Corsair are a solid choice, but fair enough if you want more power headroom if you are thinking about Crossfire.
Asus motherboards are my favourites, not sure if its worth the extra over the cheaper Gigabyte, I just chose that one to try and fit your £800 budget. Maybe try googling for reviews again.
Case is just down to preferance really, again went for the Antec 300 for a decent budget case, but the one you chose is nice
