|There's nothing stopping you from buying your own wireless N router and using it. If you were to do so, you'd simply disable the wireless on the G router, and enable it on the N. I wouldn't run N and G together as only the clients connected to the N AP would get N bandwidth.|
For info on adding a second router to your network, click on my name above in my response and read my "how-to" guide on "adding a second router". You'll want to use the "LAN to LAN" scenario for simplest setup.
I bet if you do some price comparisons, you'll also find that an N router is the same amount of money, or less, than an AP.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.