Wireless N router/cable modem combo, any suggestions?

Since the device is going to have to work with your cablemodem service, I suggest that you call your service provider for a list of approved devices.

Most ISPs, including Comcast, are starting to move away from the all-in-one Cable Gateways. It's more cost effective (for them and for you) to replace either the router or the modem if one goes bad or is upgraded. I'd personally recommend going with the Motorola Surfboard SB6120 as this supports the newer DOCSIS 3.0 standard that all cable internet providers will be transitioning to. Then you can use whatever router you like. Also, you won't necessarily see speed increase just by getting a Wireless-N router. If not all of your wireless clients have Wireless-N cards, then you won't see full speed. You will experience some speed loss by having the router be backwards compatible with the G standard that is common in majority of laptops (Wireless-N only got officially approved a few months ago). I'd recommend contacting Comcast and having their tech figure out why you're getting such a low rate. If it's hardware that was originally provided by them, they will replace or upgrade it for your at no charge. If you want absolute top performance and future proofing, go with the separate Router/Gateway as mentioned above.