5400 is perfect. I'm sure Seagate has some of their own software on the drive, so you might want to completely reformat it first, but yes, you should be able to just take it out of the case and put it in your PS3.
Just make sure to back-up whatever you want from your PS3 and be very careful not to strip the screws while replacing the HDD.
[quote name='ManuGinobili']^i believe so but why not just get a internal hard drive? i think 7200 is better for the ps3 (not a 100% sure).[/QUOTE]The original HDD that the PS3 comes with runs 5400 RPM, so 7200 is just overkill.