If its a RAM problem the video card shouldn't affect it at all. As to updates tell the shop to go for sevice pack 3, Microsoft no longer provides updates for SP2(or you could also download and install it yourself). I think it might be a RAM problem but if it works well in another system I have to say it's a motherboard problem.
PSUs are more efficient when you use 65%-85% of its max wattage, sure 80 plus certification means that it is at least over 80% efficiency at 20,40,60% levels but not as efficient as the ideal levels. Efficiency means that more power is converted to useful dc current and not wasted in the conversion process.
No I meant the PSU waste less watts trying to power the equiptment. Can't really pull up numbers but for example if you're pulling 500W from a 500W PSU(which I don't recommend) if it only has an efficiency rating of 80% at that level than it has to really pull in 625W because 20% of it is wasted as heat in the conversion process. For light use that 750W Corsair will barely draw more than 300W, drawing around 40% of capacity efficiency can't be really high at all.