Try to use two matched Ram chips and reset bios if you can get to the bios screen. You could always try adding just one chip to slot 1 to see if you can get the bios back. Is the ram all of the same speed ? 800, 667 1033 or are you trying to use different speed ram together?

What Operating system are you using? If its no Vista/Xp 64 it wont recognize your extra ram above 4gig.