It will basically be a new PC. Clean install to new disk. Old XP disk will still be in machine until all data is migrated off then it will get wiped also.
Ah, I see. I basically had to do the same thing - copied my BGT installation from my netbook (XP) to my new desktop (7) so I can play it on either one. It was very tricky to get it working because many registry entries had to be moved as well, some of which are pretty well hidden. So it CAN be done, and I was going to walk you through it - until I realized my installation came off of the CD version, so unfortunately the registry may be different.
Here's what I suggest - if you're familiar with the registry editor, search for these terms - 'baldur', 'bioware', and 'bgmain', then export any important-looking keys and import them to your Windows 7 setup. Copy over the game from the one hard drive over to the other, carefully making sure the paths match (if it's in c:\games\bg2\ on your XP machine, it MUST be c:\games\bg2\ in your W7 machine).
If you have no idea what I just said there, it's probably best you reinstall the whole thing. As for the tweaks, I'm not sure if there's an easier way to do it, but what I've always done is just run the tweak pack setup file again from your BG2 directory (setup-bg2_tweaks.exe), select Yes to see all the components, then when it asks you if you'd like to install each individual component, say no, but if it asks you to RE-install a component, still say no, but then jot down the name of that component somewhere - it means you had opted to install it earlier.
Be sure to backup your saved games directory too.
Hope that helped!