Right now I can get 4 6100 series 8 core Opterons from ebay....
Not all 61XX series 8-core processors are created equal. The 6136 processors are 2.4GHz default, versus 6128, which are 2.0GHz default. With the QRB dividend, that 0.4 GHZ makes a huge production difference, as measured by points per day. If your objective with the 8-core 61XX processors is as a place holder until you can afford more powerful CPUs, I understand. I would though, not look at 6136 or 6128 as permanent hardware. If you still want the 8-core models and don't anticipate upgrading within a few months, do not get the 6128s. The 6136s will significantly outperform the 28s, but should only cost about $100 more for set of four. The 6136s also tend be fairly consistent, good overclockers. I had mine running at 2.7GHz. (Since then I've upgraded to 12-core processors, Magny Cours also.)
Here is production data for one of my former 4X6136 rigs, work unit/time per frame/total average points, including Quick Return Bonus:
8101: 15:11, 293,603 PPD
8103: 11:10, 465,507 PPD
8104: 08:32, 460,523 PPD
8105: 11:10, 454,624 PPD
Power draw on that system was approximately 630W, full load, overclocked, in a case, with several 120mm fans. At default CPU frequency, it was about 475W, if I recall correctly.