2 sticks of ram pc wont boot

I got geforce 7050m (v2.0) with 1 x 2gb ram 800, so i decide to by another 2 gb ram, i installed it and it wont boot so i removed it and everything was ok, so i tryed the new one on its own and it works too. They both are kingston - kvr800d2n6/2g - 1.8v. My motherboard as 2 slots but cant run with both ram in..

Many times mixed RAM even of the same part number will not run together without issues. You can check your BIOS to make sure the RAM is set as "unganged" and test some more. You can also use Memtest86+ run from a floppy, CD or flash drive to be sure that both DIMMs are good. You can test one DIMM at a time and then if your PC will boot, you can test with both DIMMs installed.

Mixed means when you do NOT buy the RAM as ONE, matched, tested Kit.

The error message sounds like it's trying to down clock the RAM frequency, which usually means the two DIMMs are different internally. They can have the same part number but use different ICs on the DIMM. There are like (8) or more IC suppliers to the RAM makers and they use whatever is available that day...

You can try manually setting the BIOS latencies and voltage by booting with one DIMM then powering down and inserting the second DIMM to see if that might work?

You should probably e-mail Kingston with all the details and RAM part numbers and see if they have any other ideas.