The WD5001AALS (500GB Black) in my country (in the stores) it's about 100$ I can't afford anything above it.
What do you think about the WD640GB (32MB) Caviar Green ?
The WD Cariar Green drives are energy saving HDD's, in that they spin down when they are not addressing I/O requests, then spin up again when needed. So the spin down/spin up takes time making these not performance drives, just more energy efficient drives. The spindle motor takes 12V @ ~ 1 AMP so use 12 Watts power. probablyi 25% of that is inefficient heat, not a lot of energy saved unless you have many of them.
For performance, use the Caviar Black, for inbetween use Caviar Blue, for slower energy saving use Caviar Green.