Upgraditis is a severe, chronic disease. If you're happy with the way your system sounds, leave it as it is. Only if your needs change (ex. multi-room capability, component video switching, a much bigger rec room) should you start worrying about upgrades to your electronics.
To get a higher level of features isn't a huge investment these days (Denon, Yamaha and Onkyo make relatively cheap 6.1 models with component switching, for example), BUT for measurably better sound you're looking at companies like Rotel or Arcam...which will tack an extra grand onto the price of a receiver easily.
