If you are building for HTPC then according to your budget $500 australian when converted to american becomes $455.67
so around that budget here are some of the specs that will suffice
CPU AMD Athlon II 435 2.9 Ghz $89
MB Gigabyte GA-MA785GM-US2H $80
Patriot Extreme Performance 2gb(2x1gb) $50
WD Caviar Blue 640GB $70
Antec 3Hundred Black Steel $55
Liteon Black 24x DVD Writer $32
COOLER MASTER Elite 460 RS-460-PSAR-J3 460W $35
Total now becomes $415
you still have around $40 left that is american