After looking at the Hitachi site:

[Link: hitachi.us]

This is what I think you have...

The product has dual 181 channel NTSC tuners. The NTSC tuners will not receive HD. So even if you bought a HD tuner that would modulate the signal to NTSC, you then wouldn't have HD resolution, as NTSC will not support either 720p or 1080i.

You can choose any HD receiver that has either a Component Video or DVI output, as the Hitachi you have offers both as inputs.

If you are using the remote that came with the set, you might want to Consider a Hitachi receiver. I assume the remote from your set would operate the receiver directly, making your setup slightly simpler. If you are using a programmable remote, Your choice of receiver is limited only by the inputs your TV offers. Again, be sure to look for DVI or Component Video outputs.

If you have other devices connected to the set, be sure to check which connections are being used. If the DVI is being used by some other device, you would be better off choosing a tuner with component video outputs.