I just installed the Western Digital Scorpio Blue WD10JPVT 1TB 5400 RPM drive. Preliminary indications are that it works fine. The drive costs about $115-$125 from online retailers.
I've seen this drive start to lag or slow up in response time when you get it over half full of recordings.
The worst is when you have 2 programs recording while you are watching another recording. Picture pixels up and/or freezes. I speculate the data transfer, cache, and drive speed are accomplices in the hiccup.