So I have a few questions for you here. First of all, I have an ATX v2 mobo and an ATX v1 PSU... I've seen several posts here that say this setup can work, but it sacrifices the stability of the extra four pins. Can someone tell me what kind of issues I might face with this, if at all? The system is only running a cpu, 3 gigs of ram, 2 HDDs, and 2 fans... So I don't think there would be issues, but I defer to the experts
Second, as the title states, I'm looking to make the above as low-powered as possible, and I have a few choices of PSUs. The case it's in came with a 250W POS power supply that only has a 20 pin connector on it. I also have this psu laying around that I could use. My assumption is that even though the 750 is 80+ certified, the 250 would be better for me in this case because all I'm looking for is lower power draw. I don't claim to be any kind of expert on this, so any help would be great!
Certainly it will not draw more than the 250 can provide and the 750 although 80+ is not going to be seeing 20% load most of the time so efficiency is not that great for it either. I would use the 250 or replace it with an efficient 300-400watt unit.
If you're not going to install a discrete graphics card, then a good quality 300W PSu is more than enough for your rig. The Antec Earthwatts 380W http://www.newegg.com/Product/Product.aspx?Item=N82E168... has more than enough juice and has a little headroom in case you decide to install a GPU.
A bit of clarification for you...a PC will only draw as much power as it needs to actually run the PC. The rated capacity on PSUs are so you know that your PSU has sufficient rating to power your PC.
If you want to minimize the amount of power that your system uses, then you need to use low power components (you already do this) and a PSU with a higher efficiency rating. PSUs convert AC electricity (120-240V from your wall) into the DC electricity (12V, 5V, and 3.3V) that is used by your PC. This conversion process is not 100% efficient and some power is wasted in the form of heat. In simple terms, a PSU with higher efficiency wastes less power when converting electricity. A good general recommendation is to get a PSU made by a quality mfr with a 80+ Bronze or higher certification.