Well that would explain EXACTLY my previous situation. They had 3 Q6600's left and I was knocking up a system for myself, one for my father and one for my bro in law. All boards were Abit IP-35 Pro. The retail one I kept for myself (as you do), the thing clocked up to 3.3 on stock volts and romped on to 3.6 easy. The other two, both G0's but they were sold as 'OEM' and came in little black cardboard boxes, one wouldn't go past 3.2 and was trying to meltdown in the process, the other hit 3.35 but also was as stable as a table with two legs.
So where is a good (cheap) place to get bits from, Overclock.co.uk (not overclockers) seem to be cheap for boards but more for memory, OVerclockers seem to have the cheapest memory. I can get to Scan too, but they seem to be more procey for memory.