For what it's worth, the 48GB drive is the "older" IBM 5400 RPM drive. It's a ball-bearing drive that will become noisy within a year, but it's also the 3nd fastest drive for a laptop on the market. How do I know? We have a small fleet of laptops with these drives....bearing noise is lousy.
The fastest is the 40GNX (20gb and 40gb) IBM Travelstar models (cheaper too), followed by the Toshibia drives with the 16mb cache.
Just my $.02