A question for Experiences

Hello Dears, I have big problem i have bought a computer in specification

CPU………………..Intel …….core(TM) 2 Duo GHz
Motherboard Model ………….. GA-945PL-S3P ….version ……REV 6.6
VGA…………………………...Nvidia …. GV-NX72G512E1/GV-NX72G512E2
Operating system ……………...XP 32 bit or 64 bit

Memory………………………..RCmemory …………DDRII…total 4GB
Power supply…………………..450 watt

Bios version award (v.6 00PG )

when i make the total memory (RAM) to 8 GB its not giong to work
and i have tested all the operating systems ( XP32bit, XP64bit,VISTA 64bit) still i have the problem ? any one can tell me haw can i be able fix that?
all my respect....