I got a big speed increase out of moving from 6 GB to 12 when dealing with really large files on my Mac Pro tower. At least in the US, Apple is the most expensive possible place to buy RAM (as much as 5 times as expensive as other suppliers) - the Mac Pro is very easy to add RAM to, so buy it from Apple with the minimum amount of RAM (1 or 2 GB) you can get and add extra RAM yourself. US prices are as low as $59 for a 2 GB FBDIMM (the Mac Pro takes RAM in sets of four, so you can add 8 GB for USD 240 or go all the way to 16 (removing all existing RAM) for $480. I don't know if Apple Australia will sell you a Pro with only 1 GB of RAM (they did here when I bought mine), but that's the best way to go, because you are paying Apple's extortionate RAM prices for as little as possible, and buying all your RAM from a better-priced supplier.
Also beware of a possible impending upgrade to the Mac Pro tower in March. They may introduce a faster one, and when they do, the price on the current (already very fast) model will go way down. I'd wait to see what happens next month if you can, then either buy a faster machine for the same price or get a great deal on the current model.