If by "wow" you mean stuff like TrueHD from Blu-Ray, get the 6100 and set it for 5.1.
7.1 setups can do 5.1 with a simple adjustment in the audio menu. You can eBay or Craigslist the extra speakers, or use them as a second zone. Most of the "wow" factor systems from Onkyo and such will be 7.1.
Otherwise, the 4100 will do all right with Dolby Digital 5.1 and DTS 5.1. You can also get an Onkyo TX-SR606 standalone receiver and a good 5.1 speaker package. Might be a tad more than the 6100, but it would also have plenty of sound quality while staying 5.1.