Everything I've heard from people I talk to, they have to be exact. Model, Batch, whatever you want to call it. It is a common thing to upgrade this way down the road, just have to find the right vid card.
You could go to some of the tech sites and do some research.
http://forums.slizone.com/
www.tomshardware.com
www.gpureview.com
www.arstechnica.com