Since I need to keep using my Cable/Sat box, how can I control it with MythTV?
A few different ways, and it depends on your specific hardware and the provider. Possibilities include Serial Control, USB Control, FireWire Control, and IR Blaster (using the PC as a programmable IR Remote Control). See the HOWTO for details. At least some models of the Hauppauge PVR-150 include an IR output capability.