I wouldn't buy that system if you want to overclock, it doesn't have a unlocked processor meaning overclocking is very limited.
You'd probably be better off building the system yourself especially with the high price of hardware in Australia. Building is not difficult or complicated at all; if you can put a key in a lock and power cord into a outlet, you can put a cpu in a socket, ram in a slot and connect a 6pin power cable to a graphics card.
As for overclocking, its not bad at all. Most cpus and gpus can run at considerably higher speeds then they are sold at, but not every chip is guaranteed to hit high clocks at reasonable voltages and temperatures which is why clock speeds are very generous. The main thing is ensuring you have sufficient cooling and don't go blinding upping voltages.
For building yourself, I'd go for specs close to that system you linked, but if all you're doing is gaming then I'd go for an i5-3570k instead the i7 and a motherboard like the ASRock Z77 Extreme4.
So overclocking and cpu temps are more about your cooling solution. The airflow of a case is minimal, to not important at all. If you're using a liquid cooler like one from Kracken or Corsiar then you should be able to overclock with not trouble at all. When worrying about temps it's way more about cpu temperature than mobo temperature.
The term "gaming computers" is kinda vague. Yeah if you're buying from a company like hp or dell that don't really know what they're doing you're going to get some crap. But if you're smart about it and get a pc from a companty like maingear or velocity micro, then you know you're getting quality for your price point.
Also that one you're looking at is way too overpriced for the included hardware.
1. NCIX is a Canadian based company but ship internationally. They do a great job on assembly, and you can get a budget gaming machine for under $1000
2. CPUs get hot when then run, and run under load. Gaming doesn't require overclocking and the performance boost from it is minimal. I you're doing high res. image editing, video editing, and 3d render then overclocking will make more of a difference.
3. When it comes to building one yourself all you need to know how to do is a little minor research and when it comes to building you just need to follow directions. It's not as hard as it seems. After a quick search it looks like here's a site for down under pcs: [link]
About overclocking: So, I believe when the chips are actually produced, the clock is initially set lower than the "max clock rate" or whatever. This low clock rate is considered safe for all the chips. So hypothetically you can overclock your machine. However, it is important to keep in mind, not all chips are created equal. Sometimes there are screw ups in parts of the wafer during production. So even with chips from the same batch, your mileage may vary.
The PC you linked isn't overclocked. Overclocking usually won't help much in games. Especially if you have a high-end processor to begin with (which that computer does). Overclocking can help older/lower end processors to stay useful at the cost of increased power draw. It's not a bad thing, but it does require some special considerations.
Gaming computers come with good and bad cases, it just depends. These days most power supplies have sleeved cables and airflow isn't as much of a problem. Actually, airflow is a more or less solved problem in desktop computers and aside from exotic, custom setups... you can make basically any case work just fine. A pre-built computer is going to work by default.
You can tell if it's overclocked like this (Intel i-3/5/7): If the processor model doesn't end with k, then it isn't overclocked. If it does end with k, look up the default freq on wikipedia and if they don't match it's overclocked or just read the description, it'll say.