Dreamweaver CS4 has been a majour upgrade and I reccomend it. Its much faster and if you are into coding, there is a lot of new features that make it much easier, and the live update is a great feature!
Dreamweaver CS4 is better than CS3 not sure it's worth the cost of upgrading...but that will be for you to decide.(its a $200.00 upgrade) It has a completely new interfacewhich I like and it is easier to use.:biggthump
Dreamweaver Cs4 has a very large update on the LiveView, and has a few more coding updates and whatnot. Thats just about it. Photoshop CS4, if you're interested, really only added a few more doo-dads and added the 3D image editor.
As people have said before DW CS4 is much better then CS3 but I find Fireworks CS4 almost impossible to work with. Too many times it ends up consuming 1GB of RAM or simply crashing with an error. I love Fireworks and I'll continue using it hoping that next update will sort thing out cos I thing last one did make it a little bit better.
Photoshop is slightly better but nothing spectacular.