u don't have to use laptop 2.5 hdd, you can use desktop 3.5 hdd (cheaper & more storage) with an enclosure and set it right beside the ps3.
Yes, I would have stated the above as well but since he said he wanted to replace the hard drive, I assumed he was only after that. However, if you are interested in just adding more space and don't plan on moving the PS3 much, an external hard drive is a less expensive and probably even better use of your money since you can get larger storage that way.
Pretty much any external will work. I even used my 30GB iPod.