Again supporting what Dwarf has said.... its always good to do a fresh format when you feel it needs one... generally within a years time of use
I have been on the same install for almost 2 and half years and i really have noticed several time that my computer is getting really slow and i know i need to do a format...
I'm just a little to lazy right now to do one
To be honest unless you know your hard drive is failing i.e unhealthy sounds and continual random data loss then there is no real reason to replace you hard drive as a full format will generally recondition your drive back to the time it was new
If yet you still want to replace your drive then make sure you know what you are getting so as you don't end up replacing your drive for one that may be bigger in capacity but transfers data really slow
The best hard drive on the market to date so im lead to believe is the
Samsung Spinpoint F1 but don't quote me on that
take a moment to view the link below
Benchmark Results - Review Tom's Hardware : WD's 750 GB Hard Disk Sets New Records