SSD throughput (data transfer speed) is much much higher than a mech hdd. you know how when you load a game it takes any where for 30 to 60 seconds to load up a level...with an ssd its nearly instant. or when you boot windows it will cut you boot time down to 7-14 seconds (from the windows splash screen to log in) where as a mech hdd could take minutes.
The best use at least for me is just for the OS. I have tried games on my SSD before and really the space used out ways any real benefit you might get in faster load time. However for the OS it is like night and day. I have a total of 3 SSD's in my system now because I boot Windows 8 Pro x64 Windows 7 x64 and OS X 10.8.3 and all three boot in seconds on the SSD drives it is really a big difference.
The one thing you would want to make sure on before you install your OS on a SSD it to be sure your SATA Controller is set to AHCI mode before you install the OS. If not any real speed gain will be lost.
The biggest advantage of a SSD is that your system is more responsive, because it is so fast it work off everything before next input from your side arrives.
i use 2 128g in raid 0 on one system and 2 80g in raid 0 on my other. you may not need that much space but i suggest nothing smaller the 60g for a single drive for OS only.
Get 120(or 128). Larger if you can afford to get a larger one, but anything less really isn't large enough. I have several friends with 64Gb SSD's, and they have space issues and are constantly moving files around. Also use a guide like this: http://www.overclock.net/t/1156654/seans-windows-7-install-optimization-guide-for-ssds-hdds to help you set up the SSD and install the OS on it.