120 GXP is an IBM model and I would steer a little clear at present, they've been having rather nore trouble than I like at present despite their advanced designs.
The numbers that matter are:-
The UDMAS or ATA speed, 100 or 133. These are the speeds at which the drive eads/writes cached information.
The access time, typically about 8.5 milliseconds on modern drives.
The spindle speeds, get 7200 rpm in pregference to 5400 rpm, although see below.
The areal density. This is the number of gigabytes of data per square inch that the drive can give. A very high areal density on a 5400 rpm drive can make it almost as fast as a 7200 rpm one.
Overall advice? Go for UDMA/ATA 100, 7200 rpm. The remaining measurements are much the same for all drives. UDMA 133 is a proprietary speed of Maxtor who create that standard, it's very little faster, those numbers mean the number of megabits per second peak. In practice, once the cache is emptied all modern drives run at around 30 to 40 megabits per second.
Recommended makes? Maxtor, Seagate, Western Digital, although Samsung are now making a name for themselves and you might like to risk a toe in the water. Avoid Fujitsu, there are very few left anyway, and for the moment, IBM as said above.