You have two options one is of course a replacement drive but the other is replacing the little hard drive with a flash disk. The flash drive is nothing more than a 4gb compact flash memory card that happens to use the same connector as the Hitachi hard drive. Of the two I'd go flash myself.
The flash disk is faster and uses far less power so your battery life can increase by quite a bit. I think it is something like 10% of the power. Next to the display this is the single biggest power item. I've heard of the useful battery life increasing anywhere from 3x to about 6x after doing this. You can get the drive or flash from several places online. These are the same CF cards as used in many cameras. Most also offer the installation service.
Here is a link to one place but do a few searches and compare prices etc. Just search for lifedrive, flash, drive or lifedrive hard drive replacement and you should get quite a few.