Shop around for sure, and shop around for a long time before you go and buy or build one. The more you shop around, the better prices you are going to get.
for 300 USD I recently upgraded my computer to this:
AMD Athalon 64 x2 4800+ 2.51 GHZ dual core
Nvidia GeeForce 8600 GTS 256 mb GDDR 3 Vid card PCIE X16
2 GB of DDR2 800 MHZ Kingston Ram
and an MSI MB SLI compatable with 2 16x PCIE slots
Thats with shipping and handling without rebates ( take off close to 100 bucks with mail in rebates.
)
Then for a PC, you would need a HDD, something sata, cause IDE is getting phased out. I got a 250 gb one for less than a 100 bucks, 9200 rpm. A Monitor, I got a 19 inch LCD widescreen with 4ms response time about a year and a half ago for 189 USD, can prob get it alot cheaper now adays. Get a case. I got a really nice fancy one for 40 bucks. MAKE SURE IT FITS YOUR MB! You can get a standard keyboard and mouse for 30 bucks if you dont need anything fancy. And I have Logitech x530 5.1 surround sound speakers that I got for 59 dollars ( They dont make them anymore, but damn they are pro). And thats it. Thats all that you need for a PC. If you build it yourself, you save out on ALOT of money, rather than spending twice as much just for labor.
As for building it, its common sence. Stuff can only plug into one spot (with the execption of cdroms and hdds, in the case of IDE). The only REAL time you HAVE to crack open the MB book, is to look at where you have to plug in your power and reset switches. And that only takes a second of looking.
That rig basicly costs 600 bucks roughly. You would easily pay 1000 bucks to buy that almost exact system from a pc store, or have someone build it for you. Its all labor, and stupid EASY labor. It will take you no more than 20 min to a half hour to put it all together. And mebey an hour max to install windows, drivers and your other random things. Thats not a super monstrosity computer, but def not a pooty one. Its cheap and will run anything you toss at it. I play CoD 4 at 60-70 fps, UT3 at about the same, soldat at like 300+. Spore runs amazingly well. All of this with really decent graphics settings. I could prob even tweak it out and crank up the graphics without much trouble. Its a good price for a good computer.
And if you have a 1200 dollar budget, you can get a very very nice custom built pc. You could prob pick up a quad core, 4 gigs of ram and an 8800 GT, with money left over.
Another word of advice is, make sure, if you dont max it out with a quad and a billion gigs of ram, make sure you buy boards that you can upgrade. Boards that have a lot of extra ram support. Quad core support, and SLI or Crossfire support. So that if you want, you can just upgrade later if it isnt what you need anymore, it can save you a bit of loot in the long run.
Disclaimer!
I do not know the conversion rate of AUS dollars to USD, so prices are prob off a bit (possibly quite a bit).
Hope this helps!
Edit: Also, the amount of ram on a video card, is no where near as important as how fast the core clock, and the memory clock speeds are. If your 512 mb vid card has a 400 mhz core and a 600 ram, its not going to be that good, even if its 512 mb.
That 256 card I have, has a core of about 650 mhz ( I think, not sure) and a ram speed of 2 GHZ. Keep that in mind when you shop for video cards. Get the fastest core and fastest mem speeds, then look at ram size. But also remember: faster mem speed > faster core speed.
Oh and some links to most of the parts:
Ram:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820134012Processor:
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103212MB:
http://www.newegg.com/Product/Product.aspx?Item=N82E16813130168Vid Card:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130084Monitor:
http://www.newegg.com/Product/Product.aspx?Item=N82E16824009091Case:
http://www.newegg.com/Product/Product.aspx?Item=N82E16811144151Sorry I dont have a link to my hdd or speakers as I bought them at a physical store and not online. If you want I will search for a some hdds and speakers for you.
Note: The MB comes with 5.1 or 7.1 sound built in. No need for a sound card.
Shit, almost forgot. You will need a PSU (Powersupply) I got a 550 for about 60 bucks and its just fine. No links either, can find you one if you need it.