I don’t know anything about the Inspiron specifically, but the reason no one sells graphics cards for notebooks is that there are no “cards” in most notebooks (except for the occasional miniPCI slot with a wifi card). The graphics chip is soldered directly to the motherboard and not replaceable. You are almost certainly stuck with with the graphics chip you have unless you get a new laptop.
Actually this is not totally true for the Inspiron line of Dell. I can only speak from experience with a couple of different models. An Inspiron 1000 and my current 9300, both had a replaceble graphics module. The 1000 I know about because Dell replaced it on site multiple times under warranty.
The 9300 because I initially got it with the ATI version and latter upgraded it to a Nvidia 6800. I personally am wondering If a newer modulte will fit in my 2 year old laptop. My intrest of course would be Cuda. My guess is to look at what Dell offers for different laptops and if they offer some options that are the same on both models and yet a better option on the other model It may still work. Of course Dell would prefer to sell you a new laptop.
Yeah you actually can upgrade the video card in the Inspiron 1720 you can Pick up A Nvidia Gforce 8600M on ebay for like $140 But it will void your Dell warranty if you open your laptop so make sure that it has expired…