Read up on the Hauppauge Colossus/HD-PVR to ensure it's really what you want. There are limitations to using those devices for recording from set top boxes. One glaring limitation is that, unlike the cable card tuners, you will only be able to record a single channel at a time and when that channel is recording you cannot watch another channel. Some set top boxes limit component output (which you have to use to record from an STB with those units) to 480p so you might not be getting real high def output. Also you have to use an IR blaster for controlling channel changes on the STB and last, but not least, you'll need to use Hauppauge software (or their beta plugin for WMC).
If that fits your wants and needs, go for it.