Because the 2.4GHz processor has 512kb of L2 cache, it will probably have about twice the CPU performance of your 1.5GHz, which has 256kb of L2 cache

Not a bad way to spend the money!
If you use Photoshop and it hits the scratch disk a lot, having a faster scratch disk (and preferably one that's dedicated to that purpose only) would make a difference in that scenario. The ultimate solution is to have enough RAM that you don't need to hit the scratch disk in the first place, and the same holds for Office... if you have enough RAM, Windows will start cacheing your apps in RAM after you launch them the first time, and will re-launch them from RAM later when you want them again. You've probably noticed that after you open Word the first time from disk, it pops back very quickly when you launch it a while later, for instance.
It sounds like you do serious work with your system and it probably would be a good excuse to invest in a SCSI controller and, say, a couple Maxtor Atlas 10k IV's (or even better, a couple Seagate Cheetah 15k.3's!). They're warranted for 5 years of 24/7 operation and will definitely light a fire under anything involving the hard drive. The heavier the I/O load, the more pronounced their advantage will be. Click the
Sort button
here for perspective on how your IDE drives stack up to fast SCSI drives (the 60Gb IBM is down about 28 places on the first list there).