sorry i hit enter before i was done. Strictly from a supply perspective the 5870 will be much harder to find if you can at all in 6 months. performance is very similar with the 5870 being a little faster all around as a single card. CF is a whole different story. 6870s scale very well in CF so performance should catch up if not exceed that of 5870. 6870 is also a physically smaller card and consumes significantly less power.
and you will be able to find the 6870 for some time still, 5870 not so much
The 5870 is faster. The 6870 has a little better driver support since it's newer and more importantly the 6870 uses less power which is important if you want to have two of them. Considering how cheap that 5870 is though I would probably lean towards that, but it's up to you.
I have an i7 930 setup right now with a radeon 5830, I have a guy offering to sell me a 5830 for 75$ (so I can crossfire and have 2 5830's or he is trying to sell me a 6870 for 100$ What should I go for? I am assuming that the 2 5830's would perform better then the single 6870 right? I cant spend anymore then 100$