My setup is this.... TG/CD and NES on RF, Genesis/Sega CD and SNES on Composite.
I agree with Dan. I think you'd do well upgrading your cables. O:)
RF is a big no-no unless everything else is impossible.
Even composite is better. The TG/CD and NES should work with composite without mods (depends on which NES model, I suppose) and SNES should use s-video at least. Some people are having problems with sub-par RGB cables (including me).
The Genesis/Mega Drive should really use RGB (different cable depending on the model) and if your TV doesn't have RGB inputs, you can buy a transcoder solution (
like this one).
The order goes like this:
RGB/Component (best)
S-video (good/really good depending on hardware)
Composite (bad/passable depending on hardware)
RF (Nope.)