Should try to get a used HD5770 or possibly 6770, they go for pretty cheap nowadays and would run fine on a 500w PSU.
On Nvidia side, maybe a GTS450.
(sorry just noticed you wanted new, still, should keep those in mind!)
It will be fine to install using the proper Win 7 image that goes with your laptop. The key is tied to the hardware inside, so as long as you install the correct version of Win 7 you should have no problems activating it with the key...