black screen, no beeps, but computer turns on HELPPP

one day my computer freezes when im playing a game and listening to music.. try to turn it back on and everything powers up, fans spinning, green light on motherboard shows up, but theres no post screen, no beeps, no nothing, just a black screen.. i've tried EVERYTHING you could think of.. tried just the basic, video, ram, and cpu and still nothing.. bought a new video card (the 8600 GT pci-express) and still the same problem.. i was thinking it has to be my motherboard or CPU.. i looked at my CPU the other day and it doesnt have anymore of that thermal paste on top of it anymore, just this hard stuff.. im going to buy a part today which is gonna be a new motherboard and CPU and just get a combo that will replace the one i have one but i dont want to mess it up, so i was wondering if you guys could help choose one for me to buy thats not too too expensive.. here are my current specs on my computer (minus the new video card).. please help!.. thanks

Asus A8N-SLI nForce4 SLI Athlon 64
AMD Athlon 64 3500+ (Venice)
Corsair 1GB PC3200 (DDR400)
eVGA GeForce 6800 PCI Express 256MB DDR
Ultra X-Connect 500W
Western Digital 80GB Serial ATA 7200RPM
RAIDMAX Blue Case W/ Side Panel Window
Windows XP Pro.