I'm building a newer system over the next two to four weeks (takes a bit longer when the economy is suffering..), and I'll be using a 22 inch monitor since in my opinion, its not overkill and has a nice picture.
In your particular case, I'd keep the existing 15 inch as a backup and purchase a 22 inch monitor. You could do the leap for the 24 inch, but I've never liked it as much. - It all comes down to your finances in the end though, and personal preference.
Nothing like making the office in the apartment setup with a new high performance rig, large plasma, and a 22 inch for the computer to complete it all. ;-p