While a Mac Pro certainly would be nice, it might be difficult to cost-justify. If you have the budget, or expect in the next couple of years to expand your work with video to a higher level where you'll be working with Final Cut and in particularly with HD content, then certainly go for the Mac Pro. Unless you're working with very large Photoshop files and/or with a number of filters or go "upscale" in your video editing needs, though, a Mac mini or iMac might well work for you. When (or perhaps if) Photoshop goes 64-bit, it will be able to take advantage of the large amount of RAM the Mac Pro can hold. But otherwise, a Mac Pro may be overkill for you and, if funds are tight, more expensive than you need to go to. A decent "middle" option might be the 24" iMac with the GT120 processor and 4GB of RAM. That would give you very good performance for Photoshop, iMovie and iDVD, I think. Drop a second monitor on it for tool palettes, etc. and you'd have what I would consider to be a very cost-effective solution.
If you have an Apple Store or a Mac dealer near you, drop in an spend some time with a Mac mini and/or an iMac running some of those apps (you may not find one with Photoshop, but they'll certainly have iMovie and you can take in some of your clips to try out) and see how the performance seems to you. Again, if you have the budget, a Mac Pro would be great. But if like most pros you need to cost-justify your expenditures, consider an iMac or even a mini.
Regards.