The 6800GS is worth the £50 over the 6800GT.
Rather than get that board, get the Asus A8N-SLI (Standard), or at least take a look at it. If you get the A8N-SLI, you can then get a SATA 2 hard drive, which has a peak transfer rate of twice that of SATA (300mbps).
For example, the Samsung SpinPoint P120S SP2504C, which is 250GB.
"The P120S is hard to fault in terms of speed and noise, and it's good value for money. The 250GB Western Digital' Caviar costs almost £15 more for the same capacity, so you're not paying a premium for the P120S's performance or quiet operation. It's a win-win situation. Unless you need a truly enormous disk, the 250GB Samsung SpinPoint P120S is a truly regal choice. All hail the new king of hard disks."
There is also a 200GB version, which is just as good.
Other than that, for overclocking you can just put a divider on the RAM, so no worries there.
Oh yeah, you forogt a power supply
That's going to be an extra £50 - £60. I recommend a Tagan 480W, or an Antec TruePower. Take a look around