I did it, same thing you are talking about. SSD for OS, 1TB WD Black for everything else. I will tell you this, I won't ever change back to a regular drive again, the only change I will ever make is when SSD's become cheap enough, I am buying a large one for my programs as well. There is absolutely no comparison, once you go to an SSD, you will never ever look back. They are amazing fast, and that is all there is to it.
Noticeable? Oh yeah, you will definitely notice the difference, its huge.
Depends on your motherboard. UEFI helps a lot. Having a very fast processor also helps a lot as well. It also depends on if you have SATA 2 or 3 or even 1... It also depends on if your using a laptop or a desktop and your OS.
I found an SSD on sale (1/2 off after rebate, and the rebate came in 2 weeks! Go OCZ!) after my wife's system drive blew up, and not even a 'performance' one, and the difference was night and day. Her C2Duo went from ~10 sec to open office programs, to instant. Outlook use to take ~20 sec, and now only takes 2-3 sec. Browsers and other small programs are instant. If you have the install file on the drive, the install process is crazy fast. Plugging in a new USB device gets it ready instantly instead of waiting for Win7 to rummage around for the driver and then get around to install it. Sleep to wake is ~4 sec, Wake to sleep is ~3 sec. A cold boot is under a minute, but it rarely gets reset so I am not sure how long it takes.
No heat, no noise is also a plus.
The amazing thing to me is that this is on SATA2, on SATA3 it is supposedly much faster, but her computer is too old for that.
The down side is that they are really tiny (hers is just 60GB which is fine for an OS, browser, Sibalious, and Office), but putting large programs, and games on them is out of the question; but then again that is what the 1TB drive is for.
Be warned that there is limited success with cloning to an SSD. Some have managed to do it, but the technologies are just too different, so plan on a fresh install when the time comes. Also, there are bunch of optimizations to make in Windows to an SSD to limit the read/writes, and save space.
I just bought a mobo with z68 chip which will do SSD cashing. Meaning that everything will be on the 500GB HDD, and then the OS, and whatever programs I am using often will load from the SSD. This way I can have a single logical drive for all installs, but the stuff I am using will get a speed boost (for reading, not writing), and what is on the SSD will change to reflect my habits. That way I can go for a smaller high end SSD, and not worry so much about the space issues, but it is a ways away for me as I am licking the pocket wounds of my latest upgrade (i7 ftw!).
The thing about putting the OS on an SSD, and I put office, and few other commonly used programs like Firefox, my antivirus stuff, my email, but anyway, everything you do is instant, no waiting on anything, the OS is just damn snappy and responsive.
It's not just about boot times, (by the way, mine is about 15 seconds)
When you really notice it is like when Windows has just done an update, you know that screen where you have to sit and wait for like 2 minutes for your PC to shut down, and then wait some more while it boots up and configures its self, well forget about that! Bodda Bing, Bodda Bang, blink your eyes and it's done!