This question comes up all the time. You don't specify what model you're trying to connect, but my personal opinion is that an Indivision (either ECS or AGA) is going to give you the cleanest output, without any bulky external converter boxes or any of that cr**.
If AGA, get Kipper2K's HDMI adapter board and HDMI audio cable, then you can plug it right in to the TV's HDMI input and get video and sound in one. If ECS, you don't have an HDMI output option (I haven't tried if those VGA-to-HDMI converters work), but if your TV still has a VGA connector, it's also a pretty simple plug-in operation.
Not going to tell you what I think about plasma TV's though... blurry, low resolution, slow refresh rate, heavy-as-h*** to wallmount... don't make good computer monitors... ugh, good luck with that. 
