Windows goes on SSD, also only the most used programs go there, that is if you do photoshop 3-5 hours a day that goes on SSD, media players, browsers and common stuff is all on SSD.
Games go on data drive, they are huge and usually do not see huge improvements from it, 2 seconds less in a game startup will not change anything.
You immediatelly move Downloads, torrent music videos all to HDD, none of that fits on SSD.
Thats about all i can think of.
128 Gid SSD = 119.x gigs formated (manuf Use powers of ten, computers use Power of 2) Then You must leave a MIN of 10 % (personally I prefer 15% as a Min) unused so that trim, Garbage Colloctor, and wear leveling can work their magic. Your 128 gigs is now down to 100 Gigs.
Typical wind 7 installation, OS + programs (EXCLUDING games) is around 30 -> 40 gigs. This leaves about 60 gigs free for Games - Your choice.
For GAMES, theire is no improvement in FPS, only in the inital load and if "maps" are periodically called, they will load considerable fast if On the SSD.
also after the install you want to move windows sawp file to the data drive and if you have the ram to have your browser stop using the hard drive for it cache files. and turn off defrag on windows so it wont try and defrag your ssd,.
^ defrag is automatically disabled for an SSD during install (not a bad idea to verify).
On windows swap file (also called page file and virtual memory). The best thing is if you have say 8 gigs of ram, just go in and set the min max to 1024 mb and leave it on the SSD. With 4 gigs try 1024 mb for min/max and if you ever get an out of memery erroy then go in and set min max to 2048 mb
windows by default sets page file to 1.5 times system ram, so for 8 gigs thats 12 gigs disk space. many with 8+ gigs set it NONE. personnally I don't like setting to none, Just cut its size WAY down.
Also, take controll of restore point, over time can grow hugh. Either limit, or disable.
and. Disable Hibernation, this saves an amount of disk space equal to your ram.
Windows continually taxes your hard drive for minutia, so installing on SSD will significantly help with the pause on opening up stuff.
RetiredChief is right that in-game FPS won't be affected, however on initial loading or if the game does need to continuously draw on the HD it would show minor improvement, just don't expect any drasticimprovements.