It seems to me that RAM isn't very important for encoding DVDs or rendering. I would go for a fast processor as encoding and rendering depends a lot on your processor. Pentiums are faster at encoding DVDs than AMD processors so go the Intel route.
My experience with Macs is that they are
very easy to use. iMovie2 is the easiest to learn NLE I have seen by far (compared to Premiere, Final Cut Pro/Express and Vegas Video). It might have problems with large projects (100+ clips and edits) but I don't think you'll encounter that. The control for speeding up and slowing down clips is on the bottom and it allows you to add voice-over easily. Overall I find the program and OS very stable. There was a problem in my school where 2 groups were doing their graduation projects and they had large projects with many clips and edits and somehow their clips disappeared (not sure what happened there...). For a shorter project I did (5 minutes, not 15 minutes like the people doing grad projects) iMovie worked perfectly (NO crashes or messed up print to video).
I have not tried iDVD but have heard it's really easy to use and some people hit all sorts of bugs after they upgrade to iDVD3. You can still easily print to VHS tape. Roxio Toast might be a choice for burning DVDs. There are also some DVD recorders that connect with firewire and they encode real-time or something like that (not the best possible quality but should definitely beat VHS). see
http://www.kenstone.net/fcp_homepage/review_pana_3040_dvdburner.html
These might work with PCs too. You might just want to skim the article.
PRO
-Extremely easy to use
-It just works
-Consumer Reports find that Macs have about half the number of tech support incidents as PCs do.
-Save on office decoration :P
CON
-Might get a freakish bug??? (a lot of programs have bugs and I don't know if iMovie is particularly bad or good)
-iMovie3 seems to be a step down from imovie2 see
http://www.imovie-user.com/
(they are saying it is filled with bugs... so just use iMovie2 if you can get it)
iDVD is suspect
-The low end Macs are a bit expensive and aren't that powerful compared to PCs. I don't know how long speed changes take to render in iMovie (you could go to a store and test it out?).
-A bit hard to get more hard drive space on emacs/imacs since you can't put in another internal drive. You can get a firewire drive with a quality firewire cable but you might have to worry about fried ports if someone decides to hotswap the drive (crazy glue or a word of caution will probably fix this).
