Well I really have no experience building a PC, especially with motherboards, but I know how to plug in hard disks and graphic cards...

But I guess if I could build a gaming rig for about a $1000 AUD, i'd be happy

This is true. Like mine, which is already a year old, but it was already pretty outdated when I got it for $600 (CAD) pre-built. Yet I can still play any new game using medium to medium-high settings, if not better.

Wow that's good. Any recommendations for graphics? Nvidia sounds good, but i've been living in the 7series, so i have no idea about new GPUs. I'm not asking for an awesome one, but one that can play Civ5 or Fallout New Vegas on medium would be good...

Perfectly feasible; just had a brief look at cheap (but decent) parts and came to a total of £445 ($700 AUD).

I just took my old PC Tower out of the garage and connected it up. It's installing Windows now!

Unfortunately, it has 1GB (which is ok) but only a Pentium 4 3.0GHz processor

If I can take the case to an assembler, and give them the parts, I could probably get them to get a dual core in it

A good starting point would be either the AMD Phenom II 550 BE (decent) or the Intel Core i5 760 (better, but twice the price).

Obviously you need to check you've got the right motherboard!

My card is an nVidia GeForce 9500, which can play both Civ5 and Fallout on med-high settings at 1680x1050 resolution. You can buy it for about $60 US nowadays. However, I think the GeForce 9 series is the absolute lowest class of video cards you should buy - aim for a little higher if you can, that way it won't be completely obsolete in about 2 years.

I had a look at parts online too, and you can certainly build a computer that's much better than the one I'm using for not even $600 US (which is almost equal to AUD).

Check out these 2 benchmark sites to help you pick one out from the hundreds of products out there: one for video cards, and one for CPUs

Yeh thanks that's helpful. In the end, I wanted something affordable but good

I got these ones:
http://www.pcauthority.com.au/Review/115007,nvidia-geforce-9600-gt.aspx
http://www.pcauthority.com.au/Review/121295,sapphire-hd4850-toxic.aspx
http://www.pcauthority.com.au/Review/236773,amd-radeon-hd-6850---great-value-graphics.aspx
http://www.pcauthority.com.au/Review/151588,ati-radeon-hd-4770-is-the-best-mid-price-a-list-graphics-card-for-serious-gamers.aspx
http://www.pcauthority.com.au/Review/110020,gigabyte-nvidia-geforce-9600-gt-is-new-ultimate-mid-range-card.aspx

Right now, i feel that the ATI Radeon DH 4770 looks really good. It's $150, and apparently manages well with Crysis etc..

if anything, I like Nvidias, and if the above wasn't available I might get cheaper 9xxx series or 1xx or 2xx card (not SLI, I want stability)

I guess i'll enquire around about assembly prices. Local shops, if I give them parts, could do it for 200-300 , including payment for the motherboard and cooling...

On top of that, a graphics card of my choice +2-4GB of RAM should be fine
Thanks a lot guys

Turn out my parents don't want an assembled computer (even though i'm paying for it) and have offered to buy me a pre-assembed on, unfortunately. I'll probably buy a good Alienware one (chipping in as well)