It's an issue with OS X as well. Your hard drive is down to around 10% of its available space as free space. That will almost certainly slow things down.
Whether or not you need a new hard drive depends on how much of what's currently on the drive which can be cleaned off (deleted) or transferred to an external drive. If your HDD is loaded with music, videos, movies, and the like, it's a simple matter of moving them to an external drive rather than replacing the current 250 GB drive you have.
Once you get under the 20% mark of course it will slow right down as the drive continuously searches for free space to place items you are saving and/or downloading, including emails etc.
This is a problem with all hard drives as they become full and get below the 20% mark regardless if Windows or Mac operating systems. Replacing the hard drive in an iMac is not the easiest job in the world and maybe you should consider chscags suggestion of an external drive for music, photos etc.
As harry has pointed out, changing a hard drive in an iMac is not an easy task. For one, the entire display panel must be removed first. However, if you want a new hard drive rather than use an external you have two choices:
Thanks for the info, I have the old white iMac and have already taken it apart, That bit does not worry me. Getting the software back on the new HD is a concern.
I believe it is as easy as.
1) Physically changing the drive.
2) Booting up the iMac with the Leopard disk
3) Going to utilities, and clicking restore (I have time machine)
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