Just hooked up my friends bestBuy new ish HP w a 5770 in main 16x and a 1x16 riser connecting a 6850, after fiddling in cgMiner settings boosting fan, memory and intensity we got the 5770 to 196 and the 6850 to 140's for a combined 366 Mh/s gonna get another 1x16 riser for when the 5970 comes in and another cheap PSU.   I'm stuck w my MacPro 2,1 w x2 5770's  (280 in bitminter) and a 5970 I haven't hooked up yet bc I'm waiting on an OEM copy of Win7 or trying to bootCamp/reFIt Ubuntu w/o my 5770's flashed is not showing in boot screen? But apple logo appears.  So anyways 1x16 risers do work minus a few Mh/s as long as ur PSU is good.  We haven't unlocked or overclocked, just pushed about 75% of total power was all.  Staying on the safe side for now.  : )