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Upgrading the hard drive to a significantly faster model will perceptibly increase your performance. Your 250GB 5400RPM drive will be quite slow next to more modern 7200RPM drives.
I highly recommend going with a 7200RPM spindle speed. Because the platters are spinning faster, the drive has a lower access time, and a "snappy" system is all about access time. A lower access time will make a difference every time you do something that requires hard disk access such as opening a program or file and booting up.
In the past, 7200RPM drives used to run a lot hotter/noisier and consume more power than comparable 5400RPM drives. However today that is no longer the case. You won't be able to tell the difference in any of those areas with the right 7200RPM drive today.
I suggest a Hitachi 7K500; it is in my opinion the "best of breed" of modern 7200RPM mechanical hard drives. I also own the WD Scorpio Black 500GB (WD5000BEKT) and while benchmarks say it's faster, I don't notice a difference and it is noticeably noisier. The 7K500 on the other hand is quiet and just as fast. There is essentially no vibration from either drive. Here's a relevant review:
I also own a WD Scorpio Blue 500GB 5400RPM, and it is noticeably slower compared to both my 7K500 and the WD5000BEKT, and the Scorpio Blue 500GB is one of the faster 5400RPM drives available.
Avoid the Seagate 7200.4 at all costs; compared to the WD/Hitachi 7200RPM drives, it has poor quality and subpar performance. It has a lot of vibration and noise by comparison as well.
So in conclusion, the Hitachi 7K500 is your best bet. Samsung makes a 640GB 7200RPM drive (HM640JJ) that is supposed to be good but I have no experience with it.