Do not buy a Western Digital hard drive, you will be dissapointed. I believe Quantum or IBM makes the best 20 gigabyte hard drive. I would go with Quantum, because they're probably cheaper (and just as fast), but if you want the "IBM" name just to say you have an IBM, go ahead and buy that.
They are both great drives. Both have 7200rpm, 2 meg cache and low access times (I believe it is 8.5ms).
Hope I helped,
-RSI