i 2nd that MSI GT70. i have an older MSI GX740 and love it. the build quality isn't QUITE up to what an alienware or an asus or something else would be, but you're saving quite a bit of money by going with MSI. i've had mine about 2 years now and it still works perfectly. in most any gaming laptops, the GPU CAN be upgraded, but most of the time will require some modding to keep the newer card cool. not something most people want to get into. generally i would say dont even bother. to upgrade my 5870m to something like a 6970/6990 or better would cost 600+ just for the card, at least htats what i've found from the limited searching i've done.
so to sum it up: i give my vote to the MSI.