There are actually speed differences among 5400 drives from different manufacturers (different quality levels) though they are less significant. I would highly recommend size over speed if money is an issue having tried both a 7200 and a 5400. I was able to get a 250GB 5400 drive from Frys for 80 dollars from Fujitsu. It's been nearly as fast as the 80GB 7200 I had (seagate) and was less expensive.
If you really want capacity and have more then 80 bucks to throw at it go for the 500GB for 200 dollars (do a google search), though 250GB will most likely be plenty for a long time. Also the PS3 can use external hard drives (including networked hard drives) so for something like putting your entire movie collection somewhere it would make more sense to get an expandable external drive and put 2-4TBs of 3.5 hard drives in it then try to cram ever bigger laptop drives into the PS3 since laptop drives are more then twice as expensive on a GB for GB basis.
Also adding 3rd party components such as a bigger HD does NOT in any way void your warranty, in fact Sony encourages it (and provides step by step instructions in the manual).
PSN ID: ChosenOne feel free to add me
