Ram Boot Problem

First of all sorry if this thread has been done before, i havent seen anything on it.
Ok from the beginning. i have a MSI k7n2 delta ilsr(sumfin like that) mobo. i have athlon xp 2500+ barton. geforce fx5900 golden sample - overclocking options (needs it own power lead from psu) i have 2 cd/dvd drives 1xfloppy 1x HDD. and had 512+256 ddr pc3200 RAM (basic from computer fair)(had trouble upgrading RAM from 256 to 512, had boot problems but got it to work in the end). my psu is a basic 350W that came with my case.

i decided last week that id try the dual slot on the motherboard with this RAm i had but it didnt work. then i tried placing the RAM as i originally had it but the computer wudnt boot. so i kept changin the combinations of the ram in different slots to get it to work. managed to get it on with just the 256 in. so some friends recommended me some decent cosair RAM 2x 512 = 1 gig pc3200. when the ram turned up i put it in and it worked straight away. but my computer was randomly crashing so i reinstalled the RAM, then the computer wudnt boot. so i kept tryin all kinds, resetting cmos, cpu in safe mode etc but still no luck. the diagnostic LEDs i have on my board say ther is a memory problem, but the memory is brand new. after fiddling i get it to work with just 512 chip in now and again, and its runs fine, dusnt crash or anythin.

Does any1 have any ideas of this problem, and why i cant get my RAM to work properly. is it possibly a PSU problem, not powerful enough? since its a bog standard, free with case, 350W. im not too clued up on PSUs but i know u need to have one supply enough power. also in the bios i had the RAM voltage selected as auto and it kept changing to 2.7.. shouldnt ram voltage be 2.5 ?

thanks for any help