Computer fails to POST properly

Not long ago, I upgraded my memory and cpu. All went well until I turned it on for the first time, and it failed to POST. I reseated the memory as the beep code suggested for me to do. And it past the POST and all is well, right? Over time the computer would fail to POST again, but if you were to leave it powered up you could hear the hard drives spin down as it would restart and try to POST again. After the 2nd restart, it POSTs nearly every time. It does this with just the old memory, just the new memory, and both old/new combined (the old RAM is 2x1024 DDR2 800 Corsair XMS and the new sticks are 2x2048 DDR2 Corsair XMS).

A few days later the computer wouldn't power up correctly, the MB would get a short burst of power then shut off. I would have to unplug the PSU, plug it back in and wait 10 seconds. Once I hit the power switch it would then power the computer normally. This led me to believe that my PSU was the source of my problems. I knew it was a cheaper PSU unit and would not be surprised if it was dying.

Again after a new symptom occurred. Playing games my graphics started to get sluggish for my video card, an EVGA GTX 275. At this point I was dead sure the PSU was at fault. I assumed that card along with everything wasn't getting enough juice.

I get a hold of a different power supply to try, a Corsair 650TX. I was wrong. Two of the three symptoms persist. The computer now receives power properly when turned on every time. However, the computer fails to POST twice (with the memory beep codes) and then posts correctly. The graphics are still sluggish for games.


The current system set up is as follows:
Asus P5B-E motherboard
Core2Duo E8500 @3.16ghz
Corsair XMS DDR2 800 memory (2x1024 + 2x2048)
Corsair 650TX PSU
EVGA Nvidia GTX275 video card
2x WD 500gb hard drives
Windows 7 64 bit Home Premium

The only thing I can think of is that motherboard is at fault or the original power supply was going bad and caused an issue with the motherboard. I can't think of anything else that would cause both that memory POST error and for my graphics to be extremely sluggish.

Once the computer boots, both the BIOS and Windows7 report all 6GB of RAM. Also note that I have to turn on memory remapping in the BIOS to get have it all usable.

Does anyone have any other suggestions I can look into?

 

UPDATE: I'm not quite sure why, but I decided to swap my GTX 275 for my girlfriends GTX 285. But my computer seems to be POSTing normally and the graphics are where they should be. Also my GTX 275 is just working A-Okay in her computer as well. None of this makes sense, but I'll take it as long as its everything is working.