I know that I can use Windows Easy Transfer to move files from my old XP desktop to a new Windows 7 one which I'm hoping to get next week

, but do I have to download all my games again, like TF2, CS:S, HL2 (all of them) and all the other numerous Steam games I have, or is there some way I can copy them across? I will be moving from 32-bit to 64-bit since just about every version of 7 is 64 in a PC shop. I know I can copy the maps and sounds folders across by copying the steamapps folder, but I have loads of other games and programs like Microsoft Office, Halo: Combat Evolved (which I really don't play but I still haven't gotten to the end of yet) blah de blah too many to put here.
So what I want to know is this: is there an easy way to transfer files and programs to the new desktop, or am I better off sticking the old HDD in the new machine and upgrading it to Windows 7? If I upgrade it to Windows 7 and don't activate it can I then transfer programs across since the registry will effectively be the same?
PS. What is ReadyBoost and why does it only work on Flash Drives?