Are you looking for an appliance to interface your laptop directly to the digital cable, or interface with the digital cable box?
What company runs the digital cable system?
EDIT: This might be of interest to you.
Nope. Each chip manufacturer has their own Draft-N standard, and not all of them cooperate with each other. Once the 802.11n standard is approved and implemented, then there will be compatibility between the different manufacturers.
Who do you need to prove it to? You said your warranty has expired, so Dell won't replace it. See if you can find a replacement 6800 in the For Sale forum.
Not likely another hardware component, seeing as you're saying Windows looks OK. Software wise, it could be your game, DirectX, or the video card driver. Have you tried reseating the card?