Have you considered that other wireless networks around you may be impacting your wireless speeds? Have you conducted a wireless spectrum survey to ensure you are not on a wireless network that may also be in use by one/some of your neighbors?
As for the cable modem, you should/need to contact your ISP (which you did not identify) to ensure what you end up getting is compatible with their network/system configuration. Anything short of that will be hit-or-miss guessing where the guess may not yield a choice that works.
Microsoft MVP (2004 through 2008)