Once you are putting all that money into good camera equipment, it could make sense to up-grade from Photoshop 5.5 to Photoshop CS2 if you plan to print in a colour managed workflow starting the process from Raw files, and if you think you will be interested in learning advanced image editing techniques. The program includes the latest version of Camera Raw, which will allow you to process your raw files with ease and transfer them seemlessly into Photoshop for further work.
I don't know whether or not Adobe would give you an up-grade price moving from 5.5 to CS2, but even they don't, once you make the plunge for the new program, you are on their "bandwagon" eligible for all future releases at very attractive up-grade prices (such as $150).
We don't know what Lightroom will cost yet, but when the Windows Beta version is released you can download it for free and see whether it will do all the image processing tasks you need for the time being (it is not a substitute for Photoshop, but from I've seen so far will do all the image editing many people would need). The thinking is that the commercial version will not cost more than about 250, but again, we don't know.