Which 2x Nvidia cards in SLI is equal to Radeon 6870?

Just get the 6870. To perform similar to the 6870 by using 2 SLI'd cards is a bad idea because you introduce microstutter, bad scaling, driver issues, and you have to go with low end cards that perform terrible, especially in games that won't scale well with SLI. Get the 6870 and then you at least have an upgrade path to a second 6870. With two low end cards you end up just having to trash them when you go to upgrade.

Edit: I should just say. SLI/CF starting out is a bad idea from a value perspective. Starting out with SLI/CF will ultimately lead to wasted money when you upgrade. The only time you should go with SLI/CF out of the gate, is when you have specific needs, like gaming at 5760x1080, in which case you would want 2 top of the line GPU's to handle it.

Um gtx 550ti sli? would work or you could just save up a bit more and get a 6950

Thanks, I've decided for the Gigabyte 6870 OC, now the problem is to get a case that it fits in  

Unless its the huge 3 fan card, most 6870's will fit fine in any mid tower case.

It is the huge 3 fan card.. 290 mm, 11.4 inches....
Would you recommend any other of the 6870s?

Personally I like the HIS iceq 6870 as it has a very good cooler. The one you picked is fine though. Just make sure to look up case dimensions when you go looking for a case.

http://www.tomshardware.co.uk/forum/330634-10-case-fits...
I'm onto that aswell!!