Co2 is a decent and very convenient propellant but has upper limits on the velocity, your velocity would also fluctuate based on temperature, colder = less velocity. While I do not own a pre-charged air rifle, everyone says they are the best, 800+fps, but they are expensive, and require other goodies like a 3000psi scuba tank for filling the gun, so your investment could easily top 1000 bucks (which is why I have not gone there myself). PCP's also offer hand pumps, I dont know if you have an interest in that option.
There are tuners who make custom QB-78's and Crosman 22xx's, both powered by co2, and they are pretty fun. My Mountain Air 2250XT shoots at 710 in 85 degree weather.
You can list your airguns on AuctionArms.com, or Gunbroker.com, or the Yellow forum classifieds-
http://www.network54.com/Forum/79574/
hope this helps