I'm interesting in improving my graphics on eMachines e720 laptop, since I cannot enjoy any of games besides those for Nintendo emulator!
So, I would like an advice - are there any limitations for buying a new graphics card for my laptop, besides financial?
I am going to say no on a 99% sure assumption. The only laptops that I know of which can have graphics cards upgraded are the SUPER expensive ones that allow SLI and Crossfire. Those are ridiculously expensive.
Laptops are designed to be small and portable. They cram as many parts as they can on a single board including the video. Sometimes it is a separate card that holds the GPU, but this is still proprietary.
Gaming laptops aren't cheap. They do have some deals that you can find. I've seen some ASUS laptops that run $1000USD that have what you need for gaming, but a desktop is always cheaper, faster, and more upgradable.
Alienware machines (PC and Laptops) are overpriced in every aspect. Dell bought them and it is kind of a "premium" line now. I think their $800 base model has a Pentium, NOT EVEN A CORE2, and Nvidia M335 which I wouldn't use for gaming.
I like the ASUS gaming notebooks, they are pretty reasonable and I like their quality. They have one for $1050 with an i7 and 5730 GPU.