P4r800-vm ram

[bwlchy]

Upgrading RAM from 4x 512MB to 4x 1GB does not start boot up windows revert to old memory is fine

 

[4745454b]

Does it boot with a single 1GB stick? Guessing from model number that board has the Via 800 or 880 chipset. It might not support 1GB sticks. Might also be a setting issue.

 

[nanzer]

Maybe some bad modules? Try using 2x1GB with different pairs to isolate the bad one.
Or your motherboard/CPU/both don't support so much RAM...? Is it DDR? DDR2? DDR3?