The best piece of advice I can give to start with is to subscribe to the amigaone @ yahoo list.
RAM has been a thorny issue seemingly, the official advice is to buy registered, branded RAM, but reports have been variable about unbranded, unregistered RAM working fine and branded registered RAM not working. Again, see first paragraph :-)
Whatever you do, get the XE model, G3 or G4. Then you can upgrade the CPU (to a G4 if you want). Performancewise, a G4 will be signifcantly faster than a G3 though. Don't think about either possibly beating a relatively recent x86 chip though, it'll only depress you.
Much as I hate myself for saying this, as I'm an "nvidia man" too, get an ATi card. The 7000 seems to be the one of choice, but particular brands are a definite question to ask on the list about.
Forefront technologies AFAIK has a good reputation. Other people will be able to give better advice in this department than me though.
Lots of people have had problems with the Debian install. Again, see list.
