I just got done building my first computer and I need a new GPU because the one I have in there now (a free card that my friend gave me) is not suffice for SC2. I dont plan on crossfiring now, but I want that option open in the future and therefore I want to stick with ATI since my mobo is only crossfire ready. I dunno where to start and I would like a card that is the best bang for your buck and that can handle SC2 on high or ultra settings.
HD5770. It's not the best for your resolution but it should still be rather good. Two in crossfire will give you performance similar to an HD5870. An 550w Antec should handle crossfired HD5770s just fine BTW. http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Alternately wait a bit for the HD6000 cards if you aren't in a real hurry.
Yes, it will work fine on an 870 motherboard, or any with a 2.0 slot.
As for an HD5850 I'll quote myself from a different thread;
This is a very nice deal(crazy $50 rebate); http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Good brand, nice cooler and it's also one of only a few HD5850s that will let you raise the voltage on the core. When you do so you can often get speeds of up to 1000mhz which is a huge overclock.