AMD Radeon R9 290X - Can run GTAV, Also it's cheap let's say it is OP :P
How can you change the GPU of your laptop...

At that sort of price range I would be looking at some of the high end MSI laptops such as http://www.msi.com/product/notebook/GT72-2QD-Dominator-G.html#hero-overview (6GB 970M in a f**cking laptop
) . You can get them on ebay for just under $2000. I would never spend that much on a laptop personally though so I haven't done a great amount of research.
I can't get anything from eBay though it looks heavy and cooling doesn't seem that promising too unless I didn't read properly. :P
Build a nice desktop for $1,000, and buy an additional laptop for portability.
LAPTOP. I can't build a PC for this time and the space I have right now isn't that good for a pc..