Well, my main desktop is a 640 - like that 5150 model - and its speed amazes me during web design, video editing and music creation.
If you're thinking of the 9150, I would probably plump for the Pentium D - not so much because it will be a lot quicker, but because it is cheaper and will be quicker to some extent with the right software (which is a little way down the line)
Memory modules - fewer modules can be better if you are dealing with large quantities of data. A video editor or CFD expert, for example, would rather have just two large modules so that fewer data gets split. It is not a big difference though. Of more importance is have some upgrade room without having to replace smaller modules, though with 2GB, that's probably not a major issue either!