http://www.expresspcparts.com.au/storefrontprofiles/deluxesfitemdetail.aspx?sfid=32405&c=804100&i=239525590
For 400$ don't worry about the powersupply. You won't need it as 400$ does not buy enough components for even 200 watt. I have a 700$ system with top notch 3ghz core2duo, 7600GT graphics and 4gb ram and it only needs 120 watt max. If you have a 8800GT or so you will need 200 watt, but that card is more than half of the money you want to spend.
Take a G31 express mobo with socket 775 as it has a great upgrade path. Throw in a 2.2 Ghz core2duo or better (try if the E8200 fits in the money-picture). Buy 1 2Gb bar, most cheap mobo's have only 2 slots thus keeping 1 open for expansion.
Start with built in GMA3100 graphics and update to 7300GT for 30$ if you have that left or to 7600GT/8600GT later.
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