1) Yes, I cannot see any problems with these components.
2) The PSU is plenty.
3) The graphics card is fine the 6850 is a far better GPU than the 550Ti and you will not have bottleneck issues. I would prefer the 6870 if it were me as it is only a few quid extra.
4) XMP can set your DRAM data rate, basic timings and memory bus voltage via 2-3 switches in BIOS. It still requires you to choose an XMP profile however, and it's nothing more than an "automatic over-clocking" tool. XMS3 is another way to say DDR3 like PC3, Corsair uses XMS,XMS2 and XMS3,which are for DDR1,DDR2 and DDR3.
5) Hyper-threading makes little difference to gaming and as you are on a tight budget then this is a fair CPU,if you had the cash I would suggest the i5 2500K which is a great chip.
6) Yes the i3 will come with a stock cooler but no you will need to get thermal paste, just get rid of the wrist strap which you do not need and get some decent paste.
7) Really not sure about your case, particularly in regards to cable management. I prefer all cables to be behind the motherboard tray and only popping through to plug in to the necessary headers on the motherboard. I think you will find this is a better case all round. Case. plus its around your price mark and a good brand.
8) Is there anything you can change, already answered above.
9) If you follow the manual you should be fine,the only trouble I have had in the past is wiring the case to the motherboard but Asus makes this very simple by supplying a small plug which you can fit the case cables to and then you just plug this in to the motherboard. I used a very similar motherboard in my last build so I can help if you get stuck.
10) Providing the Vista dick is the retail version and not installed on the laptop then yes you can stick it on the new build, but you would be far better off with Windows 7.
I have been build my own PC's for a while and although it is a lot easier now things still do not always go to plan and work right away, but part of the experience is when there is a problem is working out the solution, you will learn more about the workings of a PC by building your own than you ever could buying one off the shelf. When it boots up you are going to so chuffed with yourself and will probably never buy a ready made PC again. I spend far to much on upgrades but it is like a drug always looking for the next bit of kit. Just managed to get my hands on a 27" Dell Ultrasharp at a knock down price but even so it really is bit over the top.But it needs to give my GTX 670 a proper screen.
If you want to email me when you are building or if you have problems then you are welcome to.Email.