Another 6870 - you can't spec a different card in a multi GPU setup, it doesn't work that way unfortunately. Don't select two lower end cards because of this though. Get the 6870, overspec the PSU and add another 6870 further down the line when the prices have dropped.
At the moment, not alot. One 6870 is already plenty of horsepower let alone two. In a few years, when the 6870 has been superceeded, adding another will give you a performance boost and bring it up to the level of the next generation card that replaced it.
This isn't exactly concrete, but say AMD released a 7870 in 2011...
Last question, it says i cant have the speedy delivery because some items our out of stock at the moment.
COOLERMASTER HAF 932 FULL TOWER GAMING CASE
AMD PHENOM II X4 970 (3.50GHz/8MB CACHE/AM3) - BLACK EDITION
640GB WD CAVIAR GREEN WD6400AARS, SATA 3 Gb/s, 64MB CACHE
Genuine Windows 7 Home Premium 64 Bit - inc DVD & Licence (£79)
1 x 2 METRE GOLD PLATED V1.4 HDMI CABLE (£8)
There are lots of alternatives to the items you have listed. However, most of these items are due into us in the next few days, so you could either wait a little longer or order a standard build.
Unfortunately not, but we have daily deliveries of stock. Best thing is to place your order on a standard service, unless you're desperate for the computer, in which case select alternative products.
It's not really possible to retain the same price with parts that are either equal to or better than the things already in the build. You could upgrade the parts which would cost you more money like HAF-932 --> HAF-X and Caviar Green ---> Caviar Black or you could downgrade the parts which would save more money like HAF-932 ---> HAF-922 and Phenom II x4 970 --> Phenom II x4 965. I'd just go for the standard build time though, it won't take that long.