Here's why. All the paths would be incorrect for the installed games as well as a long list of other well known issues.
If you were to move the pair of drives over as-is you would likely have the best chance of success but reading your post it appears that is not what is going to happen.
Have a GX270 SFF computer for my son and added a 160 GB hard drive which has games loaded on it and has windows XP standard. Killed the power supply as had too many things added. Bought a refurb GX270 Full tower already had Windows XP Professional on the 40 Gig hard drive. Need to take the 160 out of the SFF and put in the Full tower one. So I want to keep the XP Professional on the 40 gig hard drive but want to put it on the 160 and keep all the programs/memory. Can I really do this with the Dell XP disk and choose repair? Or, if that won't work can I partion the 160 and put the 40 gig on the partition and leave the rest on the other partition? Or does anyone have any better ideas? Thanks