I have FCP Studio 2 running on a 1.8 GHz G5 (original power pc) at home, works fine… slow, but fine.
Apple dealers don’t know anything about FCP – unless maybe he works for the Pro Apps dept of Apple – I doubt this sales guy does. Of course he wants you to spend the extra $800.
I’d make a wish list of all the other cool things you could get with your $800… if a slightly faster render time still trumps all the cool plug-ins, software, etc. you could add to your machine – then go with it.