I agree with both Mr. Peach and LethalWolfe (especially Mr. Wolfe about what's important now verses later. (BTW, I'm starting to feel trapped in a Tarantino flick)).
However, to address one of your points, a Pentium 4 will encode much faster than a G4 dual. I have yet to see what a G5 can do with encoding, but the difference in encoding speed is phenominal between pretty much any Pentium 4 and a top o'line dual G4 (we're looking at a Pentium 4 2.4 needing about 1/3 to 1/5 the time to encode the same video as a dual G4; it's just that fast) At the studio that I work at, and it's primarily Macs (close to 90% and Steve Jobs drops by to give us more any chance he gets because he's hankering to use my boss in an Apple commercial as an endorsement), we keep Penitums around just for the encoding process. I can't imagine that a G5 will be able to beat a Pentium at encoding; the reason is very simple, the encoding process has been heavily optimized to take advantage of all parts of the Pentium 4 chip whereas, the G4 chip didn't have the right extensions (this may change in the future if new codecs are written to take advantage of G5/970 on chip units.. but you're still looking at quite a bit of time before that's ready)
As for software, FCP 4 is good. I, personally, favour Avid DV. Avid DV Pro + Mojo is not only more expensive than FCP 4 but it is quite a bit more powerful than FCP4 on a G5 in speed (You might as well start comparing the Nitrus system which is way more expensive); they aren't competing in any marketplace at the moment (especially since Mojo isn't even available yet).
FCP 4 is the new kid on the block that's making some headway into the video editing world (especially since it has a much lower price) but AVID is the king and much like it's audio counterpart (Digidesign and its ProTools, which is also part of Avid the company), I don't see them loosing much marketshare to the new kids, even if the new kids are more innovative. The work flow between the two systems are similar (Apple did learn from the marketleader, Avid, when developing FCP4 which is appropriate if you're trying to break into a dominated market), but Avid still has a modularity akin to older linear editing that seems more complimentary to film editing. I don't think it's a fear of moving to a new software platform that keeps FCP4 from taking a stronghold but that those used to Avid will find some features lacking in FCP4 that they have grown accustomed to. LeathalWolfe and others who are proponets of FCP4 will probably disagree with me, but that's just my take on the situation

.
I, also, believe Avid has education pricing and you can get Avid for both Mac and Intel (FCP 4, is Mac only, of course.) A turnkey (hardware plus software package) solution for either software may be your best bet for pricing (and at least you guarantee that it will be tuned for the purpose of video editing.) Contact either Apple or Avid (if either ends up the direction your choice and ask them about good turnkey solutions for the education market; they'll both be glad to help because they "want" tomorrow's video editors working on their systems and they should be able to provide you with third-party groups to customize within your price-range (plus they know their products best; just don't ask for any comparisons between the two because obviously anything you hear will need to be taken with huge heaping spoonfuls of salt.)