Do you already have OS, mouse, keyboard, monitor and sound device (headphones or speakers)? If not they could take a big chunk out of your budget. If you just mean for the box components I read in PC & Tech Authority that the Intel i3 and i5 are the best value CPUs, in Australia anyway.
Parts to take into account:
- [~$80] Case (and PSU which needs to be able to power everthing; need to make sure there is enough space in the box for all the parts)
- [~$150] Graphics card
- [~$100] CPU
- [~$140] Motherboard (check components are supported)
- [~$100] HDD x 2
- [~$50] CD/DVD (possibly burner)
- [~$80] RAM 2GB+ (two sticks of same type)
- [~$300] 64bit Operating System
(The prices are just a starting point)
Don't forget a wireless card or network cable if you used wireless with your notebook. The next stage would be to buy a good PC mag, look at review sites and computer part store websites for your location until you've found a good balance between price and performance.
Edit: @ultifd: no way you would get a PC that good for $1000 in Australia. i7 alone is about $300. I use
http://www.umart.com.au/ and
http://www.pcland.com.au/http://www.umart.com.au/newindex2.phtml?bid=6 That one is over the budget but it comes with monitor and mouse/keyboard.