Hi From_Earth,
I see 2x1GB PC3200 on Ebay for less than US$20, so it's certainly NOT worth investing £90.
The question is whether it would significantly speed up your system. I would say probably not as much as performing a clean installation of the operating system. That's free.
Your two other questions. If you have 4GB of RAM but use a 32-bit OS, only about 3.5GB is seen by the OS. Second, your RAM will run at PC2700 speed whether you install PC2700 or PC3200.