Yeah, you can't upgrade it in MOST laptops. However, there is an increasing amount of them where you can.
Your best bet for getting a great video card in a cheap laptop is to visit the Dell outlet (
www.dell.com/outlet) and watch for exceptional deals.
Keep in mind that the faster chips from older generations will often be faster than mid range stuff from this generation!
EDIT: Just wanted to also say that you will be able to grab a 17" Laptop with a good video card for your $1250 budget but getting a 15" one with a decent video card will limit your options quite a bit more.
There is an Acer with an X1600 ( i think, it might be a 7600 lol) that can be had for around $1000. It is a 15" model.
You could configure some Dells with decent video cards for your budget as well--make SURE to use their coupons!