Can anyone help with a problem upgrading RAM.

I currently Have 512 Ram and want to upgrade to 1gbyte using a single stick.. But when I install the new stick the pc doesn't boot, when I replace the old one everything works fine:

I have ASUS P5P800 Motherboard with 4 slots (1a,2a,1b,2b.. I am able to upgrade my memory to 4g's (infact 6.5 if I wish) so I do not understand the problem.

My current stick is pc4000 DDR my New stick is Kingston KVR400X64C3A/1GB pc2300 CL3 DDR

Also am I able to use both sticks at the same time having 1.5gb.

Any help is extremely welcome.