New system troubles

Hello all, Seems everytime I search out a problem a link or two here pops up so I thought I would register and see if anyone can give me a clue as to whats going on with my new build well rebuild of a pc I put together four years ago.
Windows xp pro
Athlon 64 x2 5400+
Arctic Cooling Freezer 64 Pro
Asus M2N-E mobo
Corsair XMS2 DDR2 800 2gigs 2x1g
Evga Geforce 9600gt 512mb
Rosewill RP550 550w psu
Leftover from my previous build used what I could.
Audigy 2 se
WD Raptor 74g Main hd
WD 80g secondary
Nec 16x dvd drive
Asus 52x cd
Silverstone temjin 6 case

Initially I got it together and booted fairly painlessly. Initially noticed high cpu temps and I would get mem test failed while booting and hang on the windows xp loading screen. Installed Call of duty 4 as a test for the new machine and it looked great for about 10 seconds on max settings noticed artifacts before sound started looping and the comp essentially locked up. I realized for one I had mounted my cpu cooler blowing the wrong way the case I have has a windtunnel sitting between two 120mm fans at the base of the case and mounting the mobo upside down means the cpu and ram sit in it didnt think just being backwards should have me getting as hot as it was, when I righted it I cleaned off the little block of paste it shipped with and used arctic silver 5 thermal compound and reset it that left my cpu idling around 25c instead of the high 30's but whatevers going on with the ram is still bothering me.
I do the homework I shouldve done on the M2N-E and learn about its problems with various memory as well as only going as high as 1.95v turns out my corsair is 2.1v but after reseating the cpu cooler and changing it to ddr2 800 in the bios it seemed to be running stable enough im updating steam and decide to try cod4 single player while I wait it goes smooth enough for a second then artifacts again and before I can exit my computer blacks out it hangs on windows loading after the restart to the point I have to restart again once I get into windows Its no longer recognizing my dvd drive.
So im pretty furious I put into newegg for a rma and pull out and repackage the motherboard and the powersupply, Combo deal they have to go back together. But sitting around bored I decide to try a thing or two and put it back together before I return anything. Somewhere I read to attempt putting the ram in the two black slots instead of the yellow on the board so I do that as well as putting my sb audigy 2 se in, Initially I was using the onboard sound after reassembling it and setting everything I had changed in bios back to default boots up fine and seems very stable.
I decide to try doing some updating requiring the restarts I had been avoiding and it seems to be running like I had hoped it would and recognizing the dvd drive again. I play roughly half of cod4 single player 1600x900 res with all graphics maxed out without not as much as a stutter I decide to try teamfortress 2 with a friend whitch id finished downloading while playing cod, That runs fine as well for awhile until I attempt to change up my resolution from 1024x768 it mostly freezes though I still hear sound after a moment my screen blacks out im forced to hold in my power to get out of it, And again hanging on the xp loading screen though I havent seen the memtest fail on boot since I assembled it the second time, Trying to restart tf2 right away there are artifacts all over the screen to the point I cant read anything in the menu I manage to exit it quickly enough to avoid a reboot and now im just plain frustrated, For some reason ive no way to see the temp on my vid card but with speed fan it says around 51c I didnt think that should be hot enough to be a problem. Right before making this post I had managed to play cod4 multiplayer for a hour or so on good settings it was running perfectly until my screen got very dark, My hud and everything but what I was looking at was gone but I could still hear so I was obviously still connected I attempted to tab out to close and my screen goes completely black I cant seem to ctrl alt del close it despite the task manager appearing once then dissapearing and again im forced to hold in the power, Computer restarts hangs at windows....

Anyway sorry for being so long winded I just have no idea what the deal is here and its pretty frustrating. Returning the mobo to newegg was where I was planning on starting just because of all the bad news I've heard on it since I started looking into it. Any other guesses would be greatly appreciated though.