Good to hear that you're going to try them out: you may decide that
they're keepers (...or maybe not...it's okay either way...).
Would someone please tell me how much power I need?
If you're using a transistor-type amplifier, something with about 15+ watts per channel is more than sufficient. The better the quality of the amplifier (if it is a low power amp), generally the better the sound. Tube amps will typically sound much more smooth than most transistor amps, and you will find tube amps are generally much lower power - as little as 1.5 Watts/channel. These will work, too.
Do I need a different amp for the Heresies?
Heresies don't need any more than Khorns, but if you have a choice, I'd recommend something in the 40 W/channel capability (not that you need anywhere close to that amount of power unless playing something very, very loud and dynamic). If you find an amplifier with 40 W/channel - that's plenty.
Is there a site that will answer these kind of questions?
You're on the right forum. I don't know of any FAQ on amplifiers for Khorns or Heresies, but you should be able to search the forums using Google for keywords, and adding the following to restrict your search to this forum: "site:klipsch.com"
Make sure that you place the Khorns tightly in corners to form a seal with the corner between the left and right side of the back of the speaker - they will not sound very good if you don't. Also note that it is a good thing to place use something on the floor if it is bare wood/tile/vinyl to soak up stray midrange energy.
Also try to keep as much as possible clear between the two speakers that can reflect sound - such as large equipment racks, cabinets, or architectural details like brick masonry. Placing a little bit of fuzz material (like a thick beach blanket) on top of the speakers - letting it stick out in front of the speaker about an inch if it is rigid enough - this will also help to control ceiling bounce.
"As far as the ear can tell, consistently clean and spacious bass can be reproduced only by a driver unit coupled to a horn-type acoustic transformer..."; Jack Dinsdale, May 1974
(See profile for setup and pix.)