NF7-S and OCZ PC3500 EL-DDR Problem :S pls need urgent helpt

i just recently buy all my stuffs and as u can see i have a pair of dual channel OCZ PC3500 and i cant make them work correctly i asume its a problem wit the board cause i test them separetly and worked fine, in fact if i use both mem in single channel they work perfectly but if i use them in dual channel it gaves me a lot of errors in memtest86, the only speeds i can reach in dual channel is 200x11 free of errors in memtest, can anyone pls help me? i dont know what else to do i already tryied relaxing timmings but nothing :S rising volts and nothings :S, should i RMA my board?