In spite of its absurd warmup bug, once warmed up, Adobe Photoshop CS5 can scarf up a great deal of real memory, so that it runs far faster than CS4 does, yielding much more responsive operation. See Optimizing Photoshop CS5.
The example below isn’t a particularly big job for me; this is normal fare (many 16-bit layers from high resolution digital cameras). Fortunately, I use an MPG Pro Workstation with a dual-SSD RAID-0 stripe and 48GB memory.
Note here that Photoshop is using 10.7GB of real memory, but makes pathetic use of my 8-core Mac Pro during a Convert To Profile operation: full usage would be 800%, but it’s only using 130%. In other words, Photoshop CS5 is wasting 83% of the computing power.
This is one reason that the faster clock speed of a hexacore Mac Pro is better for most everything than a slower 12-core model: the cores are idled by software that doesn’t use them. See Shootout: 8-core 2.93GHz or Quad-Core 3.33GHz.
To learn more about performance, see Monitoring system performance with Activity Monitor.