What connections does the cable modem have? I ask because some have a USB connection, some have an ethernet connection and some have both.
If your cable modem has ethernet (which might be labelled as RJ-45, 10/100 or LAN), then it is easy to connect it to a wireless access point.
If you have USB, then I am not sure if it is so simple. I believe there are USB routers and access points out there but I am not certain.
Buy the fastest standard of wireless kit you can get within your budget. You won't see any difference in the speed at which you can access the internet, however you may decide later on that you want to use your wireless network for other tasks as well. Shifting huge files about is less painful with a faster connection.