King:
If there is only one hard drive with one partition and you install Windows 2000 and then install Windows XP, you will not be able to boot to both operating systems. Otherwise, upgrading Windows 2000 to Windows XP should not cause any problems, unless, of course, Windows 2000 was having problems before you installed Windows XP, then the problems might persist.
Regarding PC100 and PC133 RAM, in many systems they are compatible (one of my PCs can use both). However, if you install them together in the same system, you will not receive the benefits of the faster 133 "speed". Check the documentation from the manufacturer of your PC or motherboard to be certain it may use either RAM stick.
Blessings to you,
Doc