Just check the power usage. Being an old system, the battery life probably isn't that great in the first place, and as the battery ages it gets even worse. Just make sure the new drive isn't going to suck the life out compared to the old one. As long as the system supports the drive size, you should be fine no matter what. But the Hitachi 7200RPM drive is surprisingly low power for the speed, only slightly more than the Seagate Momentus 5400.2, and might even be lower power than your old drive.
Keep an eye on the price too of course.
I upgraded my Toshiba P-M 1.6GHz from a 4200RPM, 2MB cache drive to a 5400RPM, 16MB cache drive, and honestly it's hard to tell the difference in most cases, despite the increased speed AND octupled cache size. Outlook loads a tad faster, that's about it I think. I hardly see any difference in anything else, nothing I could definitely say "wow this is so much faster".