Always a hard call when it's time to Update to new Hardware.. Only you know what you really need and what your budget allows...
If portability and pci slots are not a high priority go to your local Apple store and have a look and a play with the new 24" iMac 2.93Ghz.
Even the standard ' out of the box ' configuration, seems very impressive... 4gig RAM, 640 Gig HD and the Nvidia GT 120 Graphics card... All very nice.
Two points though... Still a Glossy Screen (which some people like) and only one Firewire 800 Port, (which can be overcome with a Firewire Hub)...
Hope this helps,