Ok, 9 hours of memtest 86, no errors.
Well that's pretty much eliminated your RAM as the problem. Best to do the troubleshooting that doesn't cost you anything first.
That's great, now as someone mentioned I would look at your PSU. I have no experience with Ultra PSU's, but I haven't read anything good about them. They seem to be ranked right at the bottom. What would be great is if you knew someone with a spare supply or one that he/she wouldn't mind lending you for a day or so. It sucks to lay down a bunch of money in the attempt to troubleshoot a problem only to find out that's not it.
Your motherboard is also running a little hot, but nVidia chipsets do run hotter than Intel. You could try taking off your side panel and directing a fan into it and see if that makes a difference temperature wise and consequently has an affect on you system stability.
The cooling step maybe the easiest and cheapest step in the process of elimination.
Good luck and let us know how you made out.