It will definately take the Nvidia single card - will check later on SLI -
Yes, get a brand new one - scan.co.uk are out of stock, but go on google shopping and type Asus P5K premium and I think Tekheads and Overclock have them in stock.
with the amount you have now saved opposed to the original setup you put together the 4870x2 should well be in reach - just go for the cheapest one as they are all the same reference - some are just preoverclocked by companies and if you wanted to the ATI tool will allow you to do that at no extra cost ;)
Phew - got there in the end :D