i would cut your losses and go with a US host, its going to be 1/50th of the cost, and the only tradoff will be slightly higher latencies from your .au browsers (~100-200msec) which is fine for http/forums/whatever.
if your site isnt going to use very much bandwidth (<100gb p/m) then you could find a decent .au dedicated server - but for the same money you could get 4000gb xfer on a US dedicated
If you wanted to run games servers for .au players then you would need to look for a .au host to keep latency down
hosting in .au will be very expensive, expecially for a dedicated, and you wont get very much data xfer!
the us-au exchange rate is good at the moment, you could get a decent us dedicated for ~ 100-200 AU$pm with ample transfer.
do you really need that much webhosting power? a 3.8ghz amd? they dont even make them!