Typically we don't suggest the mac pro to people who probably won't need that much power, but since you noted in your above post you tend to hang on to computers for a long time it seems (bw g3 running 8.6! yikes!) maybe a mac pro refurb isn't that bad an idea for you. You could upgrade your monitor as you see fit when you have cash or something catches your eye, and have a machine that you should be able to upgrade to your hearts content as the years go on. (unlike the imac and a laptop... although they do have good resale value, so you could always sell if you want to upgrade like most people do)
If you don't go that route, I'd go with a macbook and then hook it up to your external and then see if you really need an imac... do you do anything graphics card intensive? Probably not if you are still runnin a g3 b/w with 8.6, unless you plan on it, because the 2.2 macbook with tons of ram will be a HUGE increase over your current set up.
You may be fine with just a regular one hooked up to your 19inch for now.
Otherwise, i'd say just a macbook pro. Personally having a second computer may sound like a good idea, and I know a lot of people who do it, but really, from my experience you end up just using one a lot more then the other (unless one is a media hub or something) and it becomes a moot point.
All the advice has been sound, but the above is just my 2 cents
Oh, and definitely wait till macworld for any of the above options. If they don't come out with anything new, the current refurbs/models will drop in price, which is always a good thing if you want to replace an aging monitor as well
