Originally posted by tjmckay 1. Get a Pentax 18-55mm f3.5-5.6 AL DA II to replace the Sigma (Can get for about $250 Aussie) and flog the Sigma on ebay; OR
2. Get a Pentax 18-250mm DA F3.5 - 6.3 ED AL [IF] (can get for $700 Aussie) and flog the Sigma on ebay. The downside/upside is that I'm locked into Pentax bodies; OR
3. Get a Canon EOS 450d with the 18-55 IS kit lens for about a $700 Aussie changeover price. Would I notice a huge jump in image quality going from the K100d with the sigma lens to the EOS 450d ?
You wouldn't gain much by going to Canon but would lose image stabilisation. A better body upgrade would be the K20D which would give you a bigger sensor and weather-sealing.
As far as lenses go, a super-zoom would not improve your IQ. If anything they are poorer, especially at the wide end. The kit Mk II would be ok, but I recommend the DA16-45. It gets you 2mm more on the wide end (noticeable), a fixed f/4 wide open, and improved IQ.
Add a second lens later to pickup the telephoto end. The DA55-300mm is a no-brainer.