Something like that, yeah!
From memory the video uses about 30% of the machine RAM capacity for video and the rest for the machine and I'd imagine the machine grabs its share of the memory first and the video takes whats left. What might be happening if you're only using say 4Mb RAM (or 1Mb if they're 256's) is that the machine is grabbing so much of the system heap the onboard video can't function. That depends of course on how much memory the System takes and that depends on how 'fat' your system folder is.
I'd certainly clean the RAM pins and connectors and make sure you put the 4Mb SIMMS into bank A and the smaller ones into bank B. I can't remember from memory which is which but I think Bank A is nearest the Hard drive caddy. It's certainly that way on my cx and it's annoying you have to take out (ideally) the caddy to access bank A. Make sure they're all the same size and speed, you can't mix them in these earlier machines, but you can use ones from different manufacturers, ideally they'd all be the same.