My gpu got fried somehow In my m6400, and when I started researching possible causes I found that a lot of people were having issues with crashing with the fx3700m. It would seem the better option is the ATI card, but is it just a simple swap?
The swap is actually pretty easy. The Dell uses a MXM slot for the GPU, you remove the old one and replace. Actually a similar process to changing a GPU in a desktop rig. Make sure you get a replacement Dell MXM GPU to minimize any possible issues.
Just keep track of all the thermal pads you may have to remove and re-use after making the swap. If you carefully remove them, you'll be able to use again. I performed a similar change with an Alienware M15x. Took just a few minutes.
Be sure to get the service manual for your M6400 to walk you through the replacement steps.