I currently have a SD DirecTV setup in my house. My question is, would it be possible for me to buy a HD receiver (such as the HD10) from ebay or such and use this with my existing card, in place of the basic SD receiver??

Most likely the card will be too old to work with it. And the H10 sucks anyway. You should be able to get a new 5-LNB dish and the H20 new from D*TV for free, or $99 at the most anyway.

You'd need the dish upgraded to receive any HD programming, I'd just get the H20 also, dish and installation is included for free in the upgrade price.

I currently have a SD DirecTV setup in my house. My question is, would it be possible for me to buy a HD receiver (such as the HD10) from ebay or such and use this with my existing card, in place of the basic SD receiver??

If I were you, I would just try to upgrade to the newest HD from D*