While I agree with most of the comments about BG being impossible, I do disagree that anyone could aoe that many characters if they have lightning shield on. I also disagree that 25 man content couldnt be boxed. Although clearly there would be some bosses that arent possible.

If it was me doing it, Id make just 25 characters, 2 paladin tanks, 1 warlock, 1 mage, 1 priest. 20 shamans. Or something along those lines. Id do it purely with the intent of killing some 25 man bosses. I think the amount of time it takes to level that many characters means that you had better have something big to motivate you at the end because you wont make it otherwise. I think this is why Prepared hasnt made it yet. Im not sure what Sam is doing but the way he was leveling his chars via grinding will take him 5 years to get to 80 anyway.

Id most probably level 5 at a time which means you dont have the expense of all 25 accounts whilst you are leveling. Also once you get your first team to 80 you have a pala tank there to boost the other accounts up to 60 with.

Once you have all 25 characters up to 80, then Id buy the equipment needed to use all of them. That way if it takes u 6 months to level all the characters, the new 8 core processors may be out or nearly out and you would be able to get a lot more bang for your buck.

If you are still 100% keen on taking the dive and leveling all 25 at the same time, Id estimate that you would want to aim for at least 4 i7 machines with at least 12gb of ram on each machine.

I hope that has helped and maybe given you a few ideas to work with