Unless the 120GB drive has an 8MB cache, don't bother. The difference in rotation speed really won't make a huge difference in overall system performance. I'd just leave well enough alone, since to do what you want would likely require a complete reinstallation of Windows and all programs.
i have a compaq 6029us XPHE SP2. i recently took a western digital external hdd wdc120 out of its external case and mounted it internaly. pc came with a seagate 80gig and i hooked up western digital as slave on same ide cable. i would like to make the 120 the master and the 80 the slave. checked in everest home edition and it shows xp as retail but the product id as oem. i want the 120 as master cause its 7200 rpm where as the 80 is only 5200 rpm. what problems if any might i run into? putting the os on a differnet hdd tells me there could be some conflict with either microsoft or compaq. any neccesary steps i have to follow to assure a smooth transition? any advice more than welcome.