After upgrading memory (256MB 133MHz) on a FIC VA503+ mother board w/AMD 450 chip the processor seems to stall on bootup. I get the "Signal Missing"
warning on the screen and have to reset the system to get a proper bootup.
The case, floppy drive and power supply are from an old Dell 486/33S but every thing else is new.

Welcome, A lot of hanging and rebooting probs lead back to a power supply unit, being either budget or underpowered for your pc power requirments. But in your case i would look at the new ram put in...check again to make sure you have the right one, plus if you already have ram in there make sure they are both same brand.

To end your problem I would either test the ram in another machine, or replace the ram with a better quality ram if you havent already!

Goodluck