Both of the above replies are correct. Depsite being about half the speed of my beige G3 PowerMac, the 0.14GHz speed difference won't be noticeable ... you might save a minute or two on the export times of VERY LONG movies in Final Cut, but then saving two minutes on say a 20 hour export isn't really worth bothering about.
Which model you buy may of course depend on what else you're getting for the extra money (more RAM? bigger hard drive?) if anything.