Unusual pc behaviour and now wont turn on!!!

i have been searching the internet for the answer to my problem but unfortunatly not found anything that gives me a solid idea, some say its the motherboard, some the cpu, some the psu and well i cant really afford to replace all 3 right now so i would like an idea as to what would be the likely candidate to replace first.
here is my story, i have had my computer for about a year now and i have upgraded parts as i see fit, new cpu's gpu's mobo's etc and until july i ended up with
mobo - asus m4a78 plus
cpu - phenom ii 710 oc'd to 2.8 running at 39C
ram - 4gb corsair xms2 ddr2 800mhz
gpu - nvidia 9600GT
hd - 500gb sata 7200 western digital
psu - xion 450w
well for my birthday i got a radeon 4890 hd and to power this i got a better psu a v600 coolmax.
i am not quite sure where the trouble started, i installed the devices and i started having trouble getting the system to get past initialising the usb controllers, i finally figured by disabling the legacy usb drivers and it started booting fine, the system ran great for about a week then while playing arma2 i heard my pc's fans go crazy all of a sudden and click off it all went.
I took my whole system apart and tried putting one component in at a time, even using my old psu to see if that would work, nothing would change, no fans would turn just one green light on the mobo would light up when the power was turned on via the psu. well i was about to buy a whole new system when the other night i noticed that my power switch connector had come off at the back of the power switch, what an idiot i am i thought. i put the system back together with my old psu and guess what, it worked or so i thought until last night when i (again tried out arma2) went through a repeat performance of the previous week. now it just sits there green light on but nothing happening, i have tried both psu's and it doesnt make a difference, i dont know what the problem is and i am up for any ideas, can you please help me?

well i would prefer not to buy a new system but due to work i need one, i would prefer to narrow it down with the help of your experts than spend all this money to find out my cmos battery was dead or something lol.

I checked out the last memory stick and it is okay. I think my problem has to do with my AMD processor memory limitation? I will be calling ASUS tech support next week and see if they can explain the situation since it appears to be over my head.