I just upgraded from a 1.67 PB to a 2.4 MB and I can tell you the difference is HUGE. I've only had my MB in service about a week but everything is much snappier. Even running Quark in Rosetta seems every bit as fast if not faster than my 1.67. Web browsing is an amazing difference, pages load almost instantly if not instantly. My fans don't come on all the time. I'm not getting beachballs. Video loads quickly and plays extremely well, no hiccups or pauses. Using my computer is totally enjoyable again.
You may want to think about a MBP, though, if you really like the 17" footprint you're currently in. I had a 17" PB 1.5ghz three years ago and went to the 1.67 15" two years ago b/c the 17 was too unwieldy for mobile use. After about a year with the 15", I found myself wishing for my 14" Pismo again. Now I'm finding (thus far) that the 13" BlackBook suits me just fine as my main machine. It's very Pismo-esque -- sturdy, gets the job done, small footprint. YMMV.