I would like to start by saying I'm not overclocking. I can get two X5670s for $280 over the course of two years, or two X5650s around Christmas for $140. Is there any notable difference between the two? I would do X5680-90s but they're like $350 just for one... Way too expensive.
Depends on what you are doing with them, The 5670 will be faster but if what you need them for is just having a mass of cores then it makes no differences.
Rendering, compiling, multimedia creation, and slight gaming.
Then you will want the faster chip.
Thanks.