The dual-xeon Dell will outrun the Powermac in just about every program/task you've listed. I don't have the numbers on me, but you can do a search online for speed comparisons. I know DV magazine did a speed test, and the Powermac lost in just about everything (not by too much though).

However, speed isn't everything. Usability is a huge factor. Do you like Mac OSX or Windows XP? Which is more comfortable for you? There are pros and cons to each OS, and it really depends on the user. I, personally, prefer OSX, but I'm equally as productive and comfortable with Windows XP and 2000.

With that said, which machine you go with depends on whether or not you need Maya Unlimited. If you want to use Maya Unlimited, then you have to got with the PC. Unlimited is not available for Mac OSX. If you're only going to be using Maya Complete, then either system will work just fine.

At that point, you need to decide which video editing package you'll be using. Adobe Premiere and Avid XpressDV are available on both platforms. Both work the same on either side. If you want Final Cut Pro, then the Mac is your only option. For the PC, there's Vegas Video and Ulead Video Studio (I think that's what it's called...).

Personally, I prefer using a Mac, but realistically, I need to have both platforms available for my work (video games). If it's at all possible have one of each in your studio.