If you want the computer to boot faster, you are going to have to get a faster hard drive. That is your bottleneck, unless you just have tons of stuff loading on startup. If you are running a slow 5400rpm drive, you would notice a difference bumping to a faster one, but the biggest increase will come from getting an SSD, like roald suggested. Using a RAM disk isn't going to help that issue a bit.
Windows 7 manages memory and swap space better than any previous version of Windows. If you have 6GB of RAM, you are not likely swapping much. If you are, then reserving RAM for a RAM drive is making your situation worse anyway, because less RAM available for applications = more swapping. Plus, unused RAM gets used by Windows to cache frequently used apps, so they start faster. Less RAM means less caching. More caching means better application startup times.