In general, SD looks better on a plasma. However the problem with plasma and videogames is the potential for IR (Image Retention). If you buy a good quality plasma set though, you shouldn't have to worry. (LCD's, while less susceptible to IR have lag issues when it comes to videogames). Just search around, I'm sure you can find a good model.
Also, with something like the Wii I'm sure its just a matter of finding a good picture setting for it...it shouldn't have worse visual quality than DVD and DVD's look pretty good on my LCD (Including my PS2 games)