Each Xbox 360 DVD drive has what's known as a "DVD key", this key is unique and paired with the console. If you put another drive in the does not have this key, then you won't be able to play any Xbox 360 games...
However you can get this key from your Hitachi 78FK, first off you'll need to get it in to mode B. To do this you need to get Slax, burn it to a CD, then boot from it - then your drive should be in Mode B. After this you need to boot in to windows, dump your original firmware and extract your drive key. You then need to spoof the firmware on it as that of the other drive. Personally I like BenQ drive's. They're not to hard to spoof, they're reliable and aren't very noisy.
Let me know if you plan on going ahead with it all... I'd reccommend flashing you 360 while your at it!
More info on Mode B, getting the original firmware and flashing it can be found here:
http://www.xplodergamecheats.net/xbo...al-needed.html
N.B. Silicon Image SATA chipsets corrupt data with the Hitachi 78FK