I have Fireworks 3 but don't use it. I also have Photoshop and don't use it. I use Paint Shop Pro. Paint Shop Pro 7 does everything that Fireworks and Photoshop does, web wise, and then some, like tubes.
I don't think Fireworks converts jpg's into vector objects. If it did, I would use that feature.
Of course, my opinion is somewhat biased because I teach Paint Shop Pro and write PSP tutorials.
If you don't have PSP version 7 , I'd reccomend upgrading before buying Fireworks.
PSPforever...mike