That's where several partitions also comes into play. The C drive on a Windows machine can be about 30-40 GB, the OS will take 10+ GB or so with a few core Programs in that folder and be very healthy if defragmented periodically. I hear the OS X guys right now, OS X doesn't need to be defragged ? Well, OS X doesn't, but if a heavier user, you'll be doing clean installs periodically so what's the difference ? Windows allows system maintenance to refresh the OS, where OS X doesn't. The D Drive, make that another 40 GB and whenever you install Programs, change them to be installed to D instead of C. After that you can partition the rest of the space to hold your data in as many partitions as is convenient. Partition Magic allows for this on the fly in Windows XP.
But the key is to have enough memory (and other resources) in any computer system,so that you rarely need to page to a hdd or even to that flash memory. The Ready Boost sounds like a wonderful feature, but it's really a bandage for not having enough memory. Yes in that example 2 seconds is faster than 11, but it sure isn't .8 seconds either. And flash memory has a limitation on how many times it can be flashed. Granted it used to be and maybe still is 1,000 times or so, but after that, it's anyone's guess whether it still works. All of this doesn't preclude a regular stick of memory from dieing either. But it's faster and really the intended piece of hardware that does it's thing.
In summary, a c drive for Core OS and Programs, a d drive for Programs & Applications and many other partitions for data. OS X people like to tout that, but really how many Apple's go out the door with anything less than 2 GB of memory installed ? How often are they upgraded shortly after. Like Vista systems, it makes no difference when the memory goes in it, you'll usually wind up with 2 GB's with these newest OS's. BTW, 3rd party memory for Macs is so much cheaper than what Apple screws you over for. I have yet to hear Apple fans ever address that fact that their computers aren't inexpensive and getting upgrades thru Apple is a prison rape scene, pure and simple.