I want to be able to run most games on at least the 2nd highest screen resolution, at med-high detail. I dont expect to play bf3 on ultra, but i do expect to play skyrim on ultra lol.
BTW the max i will pay is $140 without tax or rebates.
Because the 5850 does have a little more "oomph" than the 6850.
However, I don't think the 5850 is really in your budget, assuming purchase in the US.
Cheapest one I found is used for $150 on Amazon (here).
While the 6850 is new for $140, -$15 MiR = $125, (here).
6850 or get a cheap 6870. I bought the cheap his 6870 on newegg and its great. I am glad I did. Skyrim runs great everything maxed 2xaa and,16xaf @ 1080. Of course my 4 ghz 960t helps t out too.
The 5xxx series has a bit more "oomph" like Roark said. That being said, they are last gen, and pretty soon, the 6xxx series will be too. Get a 6850, it's a good card, at a good price, and will do what you want it to do just fine.