It depends on multiple things. As long as your drive's read/write speeds are below 33MB/s and you upgrade to a faster transfer speed you won't see a big performance jump. Its mainly the burst speeds wich gets faster. Set your IDE channel's transfer rate in your BIOS to Auto. Maybe it helps.
It really depends on the harddrive itself. BTW, if your harddrives can transfer files faster it does not mean the overall system speeds gets higher. Only loading/copying files will be faster. I don't know about the explorer.exe. Do you get any bluescreens? Maybe a hardware issue, though I am not sure. Check the cables again and its better to put only one kind of devices on one IDE cable (eg 2 HD's or 2 CD-roms)
TheMajorMMX - Intel P200 MMX @ 225Mhz - V-Tech (PcPartner) Baby-AT mb w/ Intel Triton TX chipset - Nvidia Riva TNT2 M64 w/ 16MB - 256MB M-Tec SDRAM - Still running.....