Need Suggestions

I am looking for a fairly cheap (Less than AU $400) but powerful computer that will run Mac OS X, Linux and Windows quickly -_-

Please tell me part names and details so that I can ask around in local shops. Also, newegg links are helpful

The parts I have:

• Case (Normal size)
• Power Supply (Its from 2006, Will it work on a new motherboard?)
• DVD Burner (Works with OS X)
• DVD Drive
• 80GB Sata Harddrive (Looking for something bigger)

The Specifications I would like (doesn't really matter though):

• A Motherboard With 8 USB Ports, 2 Sata, 2 IDE (The budget ones only have 1 IDE port), 2 Ram Slots, Graphics Card Slot and 2 PCI Card Slots (Intel-Socket 775: Gigabyte GA-EG31M-S2, Intel G31/LGA775/mATX)
• A Good CPU (Is a Core 2 Duo Good?) (Intel LGA 775: Intel® Core™ Duo E5200, 2.5Ghz/2MB CACHE/800FSB/LGA775)
• 2-4GB Ram DDR2 RAM: 2GB Generic, DDR2/PC6400/800Mhz
• 120-320GB Harddrive (3.5" SATA HDD: 320GB Seagate 'Barracuda' 7200.10 [ST3320620AS] - 16MB SATA II)
• Graphics Card (nVidea Preferably) (PCI Express - nVIDIA: Galaxy 7200GS 256MB 64-bit DDR2, [72SEE4HDFCXX])

Please note, I am in Australia so I can't buy from Newegg. If you know any good Australian stores (Sydney) or online shops tell me. (I don't mind if you link to newegg though)

Please post your suggestions