Also are there a lot of aftermarket upgradable video cards made for the current MBP?
You are a Desktop guy as it seems.
There is no such thing as upgradeable GPUs for more or less any Notebooks. That is one of the big reasons why gaming on a notebook is a bad idea. With a Desktop you put a new midrange GPU in after 2 years and it plays any game again at decent details. With notebooks you need to buy an entire new Notebook after 2 years and still get much worse performance while spending about 10 times as much.
There is hope on the horizon that with thunderbolt some external GPU solutions might come up which would be upgradeable. You can expect though that just such an enclosre with thunderbolt connectivity and an internal pcie x16 port will sell for probably as much as the GPU costs that you want to put in. If they do sell unbundeled at all.
For casual gaming and current games a MBP 15" works quite well. If hardcore only means you play alot but not necessarily the latest games it works. Say playing mostly CSS and stuff. If you mean you buy the latest games and want to play stuff like BF3, SC2 you better look elsewhere. It works currently but will quickly become outdated and with future games. A casual gamer doesn't care, a hardcore gamer probably does unless he/she only means to play one or two special games for years.
With a real gaming notebook the hardware stands its ground a bit longer.
Real gaming notebooks have 560M or better GPUs. Best is 6970M or something in that range. They are huge, ugly, heavy and not very mobile but quite a decent bit faster than the 6770M in the MBPs.
Better yet wait for 28nm GPU which are just around the corner and should give a healty speed boost in mobile graphics.
If you just want somewhat stylisch and fast. Maybe the Samsung 700G7A is for you. I would get a more plain and ugly looking Asus G74 though they handle the cooling better.
http://www.notebookcheck.com/Test-Samsung-Serie-7-Gamer-700G7A-Notebook.66008.0.html