What operating system does it have on at the moment? Vista?
You need to check if the processor can take 64bit if you are only running 32bit at the moment.
After that there is another issue I see popping up from time to time and that is people trying to get things working like sound and wireless network cards. As the
drivers for all of these are on the original cd that came with the laptop (if you even got one?)
You need to check if you can get windows 7
drivers for these things before you do it otherwise you might run into issues.
I think that's all that springs to mind at the moment.