$2500 AUD for a computer can get you something fairly high end, although a couple months ago I built myself a mid to high end PC for around $1300~ (including just a $16 HDMI cable, $50 wireless adapter and $170~ LED monitor).
CPU: AMD Phenom II X4 965 BE 3.4ghz - Well priced, great performance, room for overclocking in future
MoBo: ASUS M4A89GTD PRO (USB3, SATA III) - Decently priced motherboard, plenty of PCI-E slots, Sata 6gb/s, USB3
RAM: 4gb DDR3 at 1333mhz - some G.Skill RAM I picked up since it was pretty cheap and performs well
HDD: WD Caviar Black 1TB 7200RPM - I'd have gotten a Samsung F3 Spinpoint which does pretty much the same job and approx half the price although I couldn't find any in stock (brisbane floods)
GPU: AMD 6950 2gb DDR5 VRAM (Gigabyte) - Very nice card, has the possibility to be flashed to a 6970, decently priced for Australia, if I had waited a little longer I probably would have gone with an Nvidia GTX 560 Ti
CASE: Lian Li k60 - I picked this up rather than the k58 since it was the same case but $10 more got me 2 extra fans, nice looking, nice airflow, good cable management, good price
PSU: Corsair HX650w Professional - Well priced, reliable, modular, 80 Plus Bronze Certified, good leeway for power usage (ie: crossfiring another 6950)
DVD/RW: Some cheap LG lite on drive - Cheap priced, does a good job, 24x I think
I ordered all of these from Umart and then picked them up at a couple of their stores (Brisbane / Gold Coast, Stock issues since Brisbane flooding), I then picked them up the same day and built my computer. As for you never building a computer, just look up some videos around youtube, google some guides, in all reality building a computer is just like putting lego together except being a bit careful.
There is also a Umart situated in Melbourne if you want to check it out (
Umart Melbourne), it's easy to order and if you're unsure about building it I think you're able to get them to put it all together for you (that's what the guy said to be when I picked my stuff up), but other places to order parts from are able to do this I'm pretty sure.