Although I am upgrading soon to the BOLD, I had a question about HTML email viewers. On my Curve, I have Empower. Will I need to install on the BOLD?

Nope.

And you really don't need it on any Curve models, either, any longer, if you've upgraded the os to 4.5

You don't "need" it on either if you're running 4.5 OS, but Empower has alot more options for viewing/using email than the default BB program, even with HTML in 4.5. Empower however doesn't fully work with 4.5OS, and the developer is working on a release that will work with the Bold