The OP said: "I am also planning to add future digital and HD channels soon so i want to make sure that whatever card i buy will support this. "
Just to be absolutely clear, none of the cards recommended, i.e. the 1250, 2250, HD Homerun, EXCEPT for the Ceton card, will meet this demand. With Comcast, to get anything but the most basic package (which is normally the local channels + C-SPAN + Home Shopping), you need a cable card device. If you are planning on upgrading your cable package to anything but the free/basic basic cable that comes with internet, you will need a cable box or cable card device in order to view the content (such as expanded channels, and non-local HD channels, and premium channels). If you want to get that content on your PC, and you know for a fact you are going to upgrade your cable package, I would not waste my time or money with any of the suggested non-cable card tuner cards suggested above. (If you don't plan on upgrading, then you don't need a cable card, and can use any of those suggested card, and get like 20 local, basic basic channels for 'free')
1) Buy the Ceton InfiniTV. It is a bit steep in initial cost, but it has 4 tuners, which means you can record up to 4 channels at the same time, or watch a channel and record 3 more, or share tuners with extenders.
2) Buy a used ATI DCT. This is a discontinued cable card tuner that has a single tuner. You can only find it second hand. There are internal and external versions.
3) Wait a few months and see if and when the announced Silicon Dust HDHomerun Prime is released. It has 3 tuners, and is an external networked device.
4) Get an HD cable box from Comcast, and get a capture device, like the Hauppauge HD-PVR. The older external box only has component or s-video connections, while a newly announced "colossus" is an internal card and supposedly does capture over HDMI. You need to attach a little IR sensor running from the card to the cable box IR eye in order to get your PC to change channels. IR Blaster is what it's called.
Also 1-3 require you to rent a cable card from Comcast (but not a cable box), and vice versa for 4. Cable cards are cheaper to rent than cable boxes.