Before you upgrade your hard disk, some food for thought:
When Apple designed the WallStreet, they used a magnetically-actuated sleep system. While nothing is wrong with this per se, there IS often a problem with the switch's location: directly over the hard disk in the upper area of the case.
Older, slower hard disks, such as those made pre-2000, had armature magnets that were relatively weak and therefore had no effect on the sleep switch. Newer drives (particularly faster models), however, utilized magnets with fields strong enough that they clearly manifested well beyond the confines of the disk's casing (hold a ferrous screw up to one and you'll see). This would interact with the little reed switch used to trigger sleep, and, when one of these strong-magnet drives are installed, the machine would be forced immediately into a permanent, irrecoverable sleep mode directly after booting. Not every modern drive will do this, but many of them will. For example, I had a 20GB IBM/Hitachi and a 16GB Toshiba do this to me. The machine only returned to working status when the new hard drive was removed.
The only recourse: Stick with an old hard drive, and you'll be safe.
Actually, you can also choose a hard disk with a particular HDA/PCB arrangement. These drives locate the armature magnets directly next to the drive's interface connector, which keeps the offending magnetic field well away from the sleep switch.
It's kind of hard to describe without another paragraph or two, so here are some pictures:
Good:
http://www.hardwarezone.com/img/data/articles/2003/879/drive-underside.jpg
Bad:
http://www.hardwarezone.com/img/data/articles/2003/693/drive-back.jpg
I personally recommend Fujitsu (which, incidentally, the "Good" drive is). I wholeheartedly recommend Fujitsu regardless, but in these WallStreets is another application where they work better than many other drives. They're the only brand I use in these machines (if I replace the original drive, that is).