Vista Ultimate x64

My PC is an HP Pavilion a6342p with motherboard IPIBL-LB. I also installed an 1 gig Geforce 8600 graphics card which works great. It originally came with Windows Vista home premium 32 bit. However, I wanted more RAM so I bought and installed Vista Ultimate 64 bit. My power supply went about 6 months ago so I installed a Thermaltake 430 watt unit, model number TR2-430NP, but for whatever reason everytime I try and put more then 4 gigs of ram the computer simply makes a beeping noise and does nothing. I'm running exacty 4 gigs now but am disappointed because my other 4 gigs of RAM are just sitting and collecting dust.

Please help.

Thanks but realized I made a noob mistake. I tried using dual-channel RAM with standard ram. I've read that they should work together but I've tried all different combos in all different slots and for whatever reason they dont like each other. I figure I'll either get another set or save my money for a better processor. Thanks for the quick response.