for that kind of budget you're probably looking at getting either (prices are approximate since I dunno precisely how things are priced in Australia)
CPU:
intel is the pretty clear winner in pricing for gaming atm
pentium ~g620 ($60)
core i3 ~2100 ($120)
Mobo:
asrock provides great budget boards
asrock h61m-dgs matx ($50)
asrock b75m-gl (65 USD)
RAM:
get anything with below specs, you can go 2x2gb if 8gb kits are way expensive
DDR3 1.5v 2x4gb 1333+ ($45)
HDD:
not really enough for a SSD + HDD, SSDs are pretty nice for general responsiveness, but only really shorten game load times
user preference (~$50-$100) for ~500GB or 1TB HDD
GPU:
no nvidia competitors (all old tech) anywhere here, if you can save elsewhere you might be able to pick up a 7850, but not really necessary for the listed games
AMD 7770 ($100 USD) (will run new vegas max at 1920x1080)
otherwise you're stuck with something like the GTX 460 (not SE, non crippled) which requires 2 power connectors, and outputs more heat (may need a more pricey case)
PSU:
something from Antec, XFX, EVGA, corsair, seasonic, PC Power & Cooling
the corsair cx 430 is usually cheap in the US ($25) if it's not cheap there,
antec basiq 350w ($30) or neo ECO 400w ($33)
CASE:
whatever floats your boat I like something with at least 2x 120mm fans
user preference (~$50-$100)
Optical Drive:
do you even need one? DVD drive($0-$20)
OS:
this is where the cheap gamer costs
windows 7 64 bit ($100)
that would put you around $485 - $640 in the US (depending mostly on what CPU you choose)
you might want to look into picking up a cheap prebuilt with a good enough PSU (be sure to check the rated 12v amps) and enough space for a GPU (no super slim small towers) and then adding your own GPU
this path might be cheaper for you since there's usually no shortage of cheap basic computers and the heavy cost of microsoft operating systems