Good, glad you decided to keep the 2400. It's a decent CPU! If you're keeping the CPU, you're keeping the motherboard and RAM as well.
Here's how CPUs and motherboards work...
CPUs are manufactured with lotsa tiny contacts and require that the motherboard CPU socket have matching contacts for them to work together. An i5 2400 requires a motherboard that has socket 1155 and will not work with any other CPU socket. The i5 6500, for example, requires a motherboard with socket 1151 and is not compatible with any other socket. The sockets are changing every other CPU generation or so, forcing us, the consumers, to spend more, when we want to upgrade.
Intel Sandy Bridge and Ivy Bridge are socket 1155, Haswell and Haswell Refresh are 1150 and Skylake is 1151.
Considering you're keeping your 2400, there is no point in changing motherboard or RAM. You'd be buying stuff that performs pretty much how your current setup is performing - no tangible gains. Besides, once they starting manufacturing new stuff, they no longer produce the previous one, so you'd be buying 5 year old motherboard...
Get yourself a nice GTX 970, something like EVGA SSC ACX2.0+ (mostly due to exceptional customer service and warranty EVGA is known for).
Get yourself a quality PSU. Seriously, PSU is one of the most important components in your system as it pretty much can ruin every other component attached to it. You're current one is far from what I would consider "quality". The Watts of the PSU says nothing about the quality of the PSU and more W does not mean better. 500W is sufficient for your system. I'd look for something that is 80+ Gold certified, around 500 - 550W, ideally you should check reviews of the PSU. JonnyGuru's website is pretty much my Nr.1 authority when it comes to PSUs. If a PSU gets 8+/10 from them, it's solid. I haven't checked the prices in AU for PSUs, but I'd say expect to spend aroun ~150 AUD for a good unit.
Since you say you don't know about cooling, I'm assuming you have an Intel stock cooler on your CPU. They're not "bad", but they're far from being good. They don't cool too well and they are loud. Treat yourself with a decent aftermarket CPU cooler. Cooler Master Hyper 212 EVO is a good cooler for a good price, but there are a lot of other options too.
Check this thread here:
http://www.overclockers.com/forums/showthread.php/748535-Recommended-Cases-of-OCF
What sets apart a case and a good case? Quality materials, good airflow, good design (not just outside, but inside as well). When shopping for a case, make sure that the cooler you're getting will actually fit - some of the smaller cases for mATX builds, for example, won't be able to accommodate large 140mm fan coolers!
And last but not least (perhaps the biggest performance gain!). Get an SSD. Seriously, it's one of the biggest upgrades you can and should get!
Hope this helps!
Sorry for the typos and spelling. Too lazy to re-read and correct them... plus, not a native English speaker, lol!