Unplug power, pull all the add-in cards, and use the motherboard jumper/whatever, to fully reset the bios. Re-add the gpu (assuming it isn't an onboard unit and could be removed in the first place), power up to bios, load safe/load optimal, and go from there. (please note that it is important to actually pull all the cards from the machine for this step, and then re-add them afterwards... I recommend the gpu first, and then once you get it working without any problems, start putting any additional cards back in, one at a time)
For your ramtester, I prefer http://www.memtest.org/
, and remember to test one stick of ram at a time, so that you can firmly identify any troublesome sticks.
Check the contacts on your ram, and on the motherboard, clean with alcohol, etc. Sometimes you get corrosion, especially if they cheaped out and didn't use gold contacts for both. Also sometimes you get foreign material down in there. Cpu will usually be ok from this ailment due to the nature of the socket.
I'm betting it's the first, and your boots are hitting trouble when they hit the pnp.sys phase of things. I've seen that, specifically, before.