Honestly, I'd say pick up World Party, for all the new weapons and such. The PS1 version is quite good, but with low-res graphics, and the Dreamcast version is even better. The NGage version is a really good port, considering what they were working with. Almost everything made it intact. It ended up pretty rubbish on the GBA though.

Or you could just wait for Worms Open Warfare on the DS and PSP in a couple months.