Boot may be marginally faster, however in real world usage I am not sure that your system will feel much faser.
With only 4GB or RAM, you will be doing swapping, so a fast disk will give some benefit, however avoiding swapping will have a greater effect.
What is your memory usage under your normal load?
Are you using 64bit Windows or 32 Bit windows. If it is 32 bit then you can't use more than 4GB of RAM.