1. Athlon II x3 is currently the sweet spot. $70 and for amazing performance.
2. Ga 790 ud4
3. Yes if it's got sufficient +12v rail amperage.
4. No, if you plan to max out settings and AA. Quadfire 5870's might, but I haven't seen any bench's yet.
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Reading that first question had me confused for a second. AMD uses x? for the number of cores on the CPU and II for second generation. So it should read Athlon II x3 435 or Phenom II x4 955. To answer your questions:
1. The 435 x3 OC'd shouldn't significantly bottleneck a single 5850. Look at the chart, the 5850 will be between the 4890 and the 4870x2. The tri core 720 BE doesn't hinder that at all. The lack of L3 cache may hit performance 5% or so. The 955 will be easier to OC and certainly won't effect the GPU. The other side of this equation is if you're looking at CF down the line, then I'd get the 955 as that might be too much for just a 435.