Step 1.) Pick the device:
You still have 3 options here.
First option is the Ceton InfiniTV 4 USB. It is now available on Amazon.com for just $299.99. While it is the most expensive option, it simply connects via USB and is only device that will allow you to record 4 channels at the same time.
Second is to go with a network attached tuner. This is a device that connects to your wired network via ethernet. These are great because you can put them anywhere in you house, and you can share them between multiple HTPCs in the house. The obvious choice is the SiliconDust HDHomeRun PRIME which offers 3 tuners built in. This will allow you to record 3 shows at the same time!
The second option here is to go the USB route.
Another option for USB is the Hauppauge WinTV DCR-2650 Dual Tuner Cable Card TV Tuner. Hauppauge is a trusted tv tuner maker that has been in the game for some time. This is a dual tuner which means you can record 2 shows at the same time. It is a bit cheaper than the HD Homerun Prime and will only run you $150. You can read a full review over at The Digital Media Zone.
Step 2) Get a cable card
You will need to call up your cable company and get a multistream cable card. You can usually rent these from your cable company for about $3-$5 a month (you should get 1 for free). This is still a savings over the $10+ that you pay to rent a cable box with a horrible interface.
Step 3) Cable card install.
You will literally need to have the cable card installed into the new device. You are now allowed to do a Self Install, however you can simply have your cable company come out, and type in some codes to handle this. Check out a nice article with pros and cons over at The Digital Media Zone.
Inside of Windows Media Center you will want to run the "Digital Cable Adviser" in the extras gallery. After your Revo passes this test you just run through the TV Setup again.
Step 4) record awesome television Just remember that you will not get "Premium content" like cable on demand or VoD. However now you have an awesome interface with Windows Media Center.
I hope this was helpful. If you have questions feel free to contact me.