Whatever you do, if you want HD you will be much cheaper to have Directv install and give you the receiver. The regular HD is free, the HD-DVR is $99 for most customers in your situation. For some it is $99 for HD, and $199 for HD-DVR. This will be a new 2 year contract.
Call 800-directv and see what options they can offer you.
If you want just another SD receiver, it is either free or $49 depending on your situation, and will add 1 year to the contract.
If you purchase a receiver and install yourself, you might have to get the multiswitch like charper mentioned above.
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