If you look at my signature, you can see that I'm ordering an Inspiron 9300. It looks like the Inspiron that you're considering is the XPS Gen 2 (the only one with the P-M 2.13GHz option), which is basically the luxury version of the i9300. The XPS Gen 2 actually only has about 2 hours of battery life, because it has the 256MB Geforce 6800 Ultra Go video card, which is what makes the XPS Gen 2 the top laptop for gaming and a solid laptop for 3D work. If you're a gamer, then I would say go with the Inspiron 9300 or XPS Gen 2 with the Geforce cards FOR SURE, especially since they're running Windows.
I don't know how Photoshop CS runs under a Pentium M platform versus a G4 platform. In terms of the screen, the XPS Gen 2 does come standard with the WUXGA screen, but the whole i9300 and XPS Gen 2 line (they use the same WUXGA screen) has slight quality issues with the screen in terms of light leakage from the bottom and a "sparkles" effect on the screen (which has been more accurately defined as looking like a light greyish metal dusting or sheen on the screen). These effects seem to vary from screen to screen, though, with some owners saying it's fine and barely noticeable, to other owners saying that their WUXGA or WXGA+ is really messed up for them. Generally, though, I'd say that if you want the WUXGA resolution of 1920x1200, then the screen for the XPS Gen 2 should be just fine. Most owners say that they love their screen, I think. Anyways, for US customers, you can always return your Dell laptop at no cost.
With a Pentium-M 2.13GHz and 1GB of DDR2-533MHz RAM (it's also dual-channel, but that's not necessary at 533MHz), along with a 100GB 7200RPM drive, that's a very fast machine. In terms of very heavy Adobe Photoshop CS tasks, I'd expect it to outperform the Powerbook. However, I don't know if most ppl are really pushing their Photoshop CS that hard during their average use.