I'd recommend getting the component cables if you have an HDTV or an SDTV that takes them, if not get S-video if you're SDTV has that port.
A classic controller if you're going to play SNES or Genesis games (GC controller is terrible for these with the small d-pad and retarded button layout), a wavebird and GC mem card if you want to play any GC games.
As for Wii games, only Metroid and Zelda are must buys at current prices IMO. Super Paper Mario is ok, but didn't feel worth the $50 price tag to me (I got it for $39 acutally, and sold it for $35, so about right).
RE4 is definitely worth the money as well if you never played it, I played the GC version earlier this year before the Wii port was announced.