Thanks to you all for your input. The prices recently dropped on ATI in Australia so the gap is down to $33 (between the two cards in question). I now agree the 4890 is the way to go even if it doesn't have the better cooling system. My case should have good enough cooling, regardless.
I must admit, I've been starting to read about Nvidia cards... Looking into things like the 285GTX (as a powerful, cheaper alternate to the 4870X2) but think I want to stick with ATI. From reading, Nvidia seem to have less noisy fans and less power consumption than ATI (generally speaking). The game that's most important to me is Operation Flashpoint 2 (not yet released - quite sad how keen I am! LOL) and this game uses Codemasters EGO engine which is the engine they used for Race Driver: GRID and ATI seem to smash Nvidia in all the tests I've seen involving that particular game.. Kinda like how NVidia always seem to smash ATI when it comes to Crisys.
Only question now is; By the time I'm ready to order my parts, will a 4890X2 be released which will bring the cost of the 4870X2 down to my budget
I hope so, but if not I'll be happy with 4890