Hi TS, I would suggest you convert your 450d into an IR camera then buy the 1DsMk3 + get the f/2.8 trinity plus prime L 35mm, 50mm, 85mm, 100macro, 300mm..
Try getting one picture everyday and think of way to improve the picture taken. Post in the critique section if you think it will help to gain more insights.
After awhile, you will realize that it's no longer possible to improve it without having an addition to your gear... then you know what to buy
i can only think of 18-200 for the longer range, but IQ is pretty much the same as 17-85. $800 won't get u a better lens than 17-85 IMHO. the replacement of 17-85 is the 15-85 which excels it in IQ, but the range remains almost unchanged. the price is $1.1k. 17-55 has shorter range, costs $1.5k, even if it is used - it's about $1.2k. maybe you can look into third party lenses.