First, never buy a set of headphone/headset with more than two drivers.
OK. Is this for PC? I'll assume it is. Are you afraid of the inside of your PC or do you have a laptop? If you are or do then track down the Gamecom 777, it is discontinued but there are plenty still in shops. From reviews it is slightly better built that that 780 and at least has the option of plugging into a proper 3.5mm jack. I got mine (from Officeworks if I remember correctly) for about $70. If you have to have a USB headset, or indeed a "gaming" headset, just don't spend a lot of money. None are particularly good at the budget you are looking at. The Sennhieser PC350's are OK but still not great for the $250 you'll pay.
If you aren't afraid of the inside of your PC and you don't have a laptop, you should buy a cheapish sound card. The Asus Xonar DG is $32 at MSY, it is PCI but you can get a PCI-e version for about $35. Check staticice.com it is a good place to search for PC/gaming/electronic stuff in Australia. Already with my ho hum 777 and the DG I get a large improvement in positional audio over the 777 USB surround adapter. The 5.1 sound is also an improvement over my mobo's onboard Realtek 5.1. Not many games still use EAC but if you play ones that do it might be worth getting a Creative card as EAC is superior to Dolby Headphone if you have the option.
Then it comes down to headphones, not headsets. Everyone seems to recommend the ATH-AD700 as having the best positional audio for FPS games, although many also say it isn't much "fun" with anemic bass. The cheapest you'll find it in Aus is $150 at PCCG so it's out of your budget anyway. Instead I'll suggest the Samson AR-850. Not available retail in Australia, but you should be able to get it for about $60 shipped from the UK via the online auction website. Apparently it is almost as good as the AD-700 for positional audio but it has much more bass, so games, and music, will feel more "fun". They have a high clamping pressure, so may need loosening (as will the Gamecom) and vinyl ear cups so they will get sweaty, but you can do a relatively cheap "sock mod" to them that should alleviate that (or buy $60 AKG ear pads that are direct replacement but cost more than the phones themselves).
Buy a cheap desktop or clip on mic for VOIP, TS and the like.
I have the AD-700s and while they are super comfortable and terrific for positional audio the lacking bass does make single-player games and genres other than competitive online FPSs a little disappointing. I've got some AR-850's being shipped at the moment, I'll see if they completely replace the AD-700s or I end up having a BF3/Tribes/Planetside 2 set (AD700) and a movies/music/every other game set.
Few. Lots o' words there. Anyway, check-out Head-Fi to get even more recommendations and to get even more confused.
