If i remember correctly, you cannot put 7200 rpm drives in ps3, they drink too much juice.. now i'm not sure if this was only for the slim model, or some specific models, so i could be wrong.. but... since i plan to to the same thing soon.. i prepared a few things.
firstly, 2.5" hdds cost a lot more than their 3.5" counterparts..
so i bought an external housing, big, black, shiny.. bought me some extension cables from ebay..cost a few $ since the ps3 is placed so the cables wont be showing, the housing will fit nicely between the black xbox360 and ps3 i plan to buy a 1tb hdd, connect it with data cables in the ps3, connect power from the external housing and have me some fun..
but.. if this is just a bit too much for you i think that the biggest one you can buy is around 640gb. my advice would be a western digital or a hitachi, googled a bit on that 7200 issue, there were a few posters that claim their 500gb wd work at 7200 without issues, also there were mentioning the length somewhere around 9.25" or something