I've got a 20'' upgraded to a 750 GB HDD. It's a Seagate Barracuda, and I'm lazy, so enjoy some copypasta:
Key Features and Benefits
SATA 3Gb/s interface
Perpendicular recording technology for maximum drive capacity and reliability
16-MB cache buffer
Ultra-fast performance
Superb reliability
Whisper-quiet operation
Enhanced G-Force Protection against handling damage
78 MB/s maximum sustained data transfer rate
Clean Sweep calibration and Directed Offline scan diagnostics
RoHS (restriction of hazardous substances) compliant
5-year warranty
Additionally, it's a 7400 RPM drive with 4.16ms latency.
Now, I've read somewhere that Western Digital drives are being used too, but i do not know what models.