I've been looking at all the reviews and am considering the Asus P6T Deluxe V2 Intel X58 motherboard but only for its overclockign ability. Otherwise consider the Gigabyte GA-EX58-UD3R.
You'd possibly be looking at well over £200 for 12GB RAM. I would be probably be looking at 6GB of Corsair RAM, Dominator in preference, mainly for the lifetime guarantee (which is honoured). However to give overclocking headroom I would also consider G.Skill Trident 6GB (3x2GB) DDR3 PC3-16000C9 2000MHz Triple Channel Kit (£150).
Cases are a lot to do with personal preference. I really like the Silverstone TJ07B-W V2 click here but it's pricey and not really that good for aircooling. For watercooling it's brilliant. The Silverstone TJ10B-W is spacious, looks good (to me anyway) and provides good air cooling click here
I highly rate the Be Quiet 650 Modular PSU but it wouldn't be enough to power a Crossfire configuration. It's extremely quiet and comes with loads of modular cables.
Rather than Crossfire you could consider waiting until the X2 version of the 5870 is released. I've used Crossfire in the past and found that if you wait a while that you can get the latest generation of a single card that outperforms the 2 cards. Also 2 cards generate a lot of heat.
You will be limited to some extent of what the system builder can provide though.