Also.... With the floor speakers... would I actually hear a noticable difference? IE fuller range? bigger thump? Better clarity? I am not exactly not satisfy with my onkyos... but once again just want something new and "better" without breaking a bank
I think you'll get much better sound because you're getting a better speaker, Athena vs Onkyo, not because you're switching from a bookshelf to a floorstander. There are alot of really excellent bookshelves out there. I recommend you replace all three (L/C/R) as a set with the budget you have. Your receiver will still power them fine and the rears will still sound good.

Rears are are generally not stressed in a surround set-up, but would be if you use alot of SACD or DVD-audio disks.
If you want bigger thump, sink your budget into a decent HSU, SVS, Velodyne, or Outlaw subwoofer and wait on the rest.
you might also take a look at the Bic America speakers at
www.thetwistergroup.com
I had one of their centers for three years and it had excellent sound (heavy bastard, though). You could put together a quality set for about $400.