Powerpoint's not resource-intensive on the presentation side. Pretty much anything you buy will be fine. If XP, 1 GB of memory will be OK. If Vista, 2 GB. CPU-wise, again, bottom end will be OK, but no reason to choose a system that's Core Solo these days. One of my machines is XP Pro, 300MHz CPU, 640MB RAM and it's just fine. If you're going with Dell, any of their end-of-line discounted offers will do.
If you don't actually use Powerpoint when making the presentation, but package the presentation, that will run fine on the most ancient machine you can find.
Presumably, since the presentations already exist, you can simply run them on existing kit and see if the performance is acceptable, and use that as your guideline.