Get a Gigabyte board. Asus are recommended as much as Gigabyte these days, but Gigabyte have a better website for downloading drivers and getting info, so that's the 1% extra. I've got a fairly fancy P35-based motherboard right now that cost about $150ish, but you should be able to get a decent one for $120 or less. In terms of selecting a motherboard, you might want to take note of the number of hard drives in your computer that you want to keep, as the newer motherboards have more SATA hard drive plugs and only one of the older ribbon cable ATA plugs.
You're probably going to have to get some new RAM, but that's not too expensive, you can get 2GB for about $50. A cheap Core 2 Duo can be found for under $100. An educated guess would say you definitely need a new case and new power supply unit for all this new hardware. Cases start from around $50 and you can get a decent case with a PSU included for as low as $70.
That of course is just to get you to the point of considering a new graphics card! There's a lot of scope in there, but I figure if you haven't had to upgrade yet, you're not looking for a video card that can render games like Crysis in maximum detail. I would say you could be looking at a $200 card max, something like a Geforce 9600GT, which might be as low as $160.
All in all, it's about a $500 upgrade, but you could probably keep trimming costs if that doesn't float your boat. It helps a LOT to have a friend who knows exactly what they're doing, because that can save you a few mistakes.
Some good places for info
PCPP regularly updated new PC thread (good for quick what-do-I-buy? decisions):
http://www.pcpowerplay.com.au/forums/showthread.php?t=97647
excellent search engine for Aus computer hardware prices:
http://www.staticice.com.au
A few reputable hardware retailers in Aus
http://www.msy.com.au/ (not online, but match online prices)
http://www.centrecom.com.au (several stores in Melbourne)
http://www.pccasegear.com (lots of gadgets and geek chic, Melbourne based)
http://pcmaniacs.com.au (Melbourne based)
Many more can be found listed at
http://whirlpool.net.au/wiki/?tag=Australian_PC_Shops