New build problems memory

ok so I have a brand new build that constantly beeps and wont turn on, 1 short beep then 1 long beep which indicates a memory error. I have 2x2gb ddr2 ram and it always beeped until I only put 1 stick in. then it worked. after a couple of weeks I encountered a blue screen of death then it happened all over again. and I tried moving the memory and it worked again when I had both stick of ram in the computer. then once again a couple of weeks later the blue screen and then beeps. so I left it alone for about a week then I tried to turn it back on and it only worked for 5 minutes then crash. And everything in the system is new except the hard drive.
any ideas on what's going on?
Build:
Amd Athlon 64 X2 5600+
2x2gb ddr2 memory
Nvidia Geforce gt 640
mcp61pm-hm nettle 1 motherboard ( http://h10025.www1.hp.com/ewfrf/wc/document?cc=us&lc=en... )
320gb 7200 rpm Seagate hard drive