Telephoto:
55-250mm IS(around $285)- has received pretty good reviews from owners and sites.
70-300mm IS(around $600)- Is better built, faster focusing, and longer telephoto than the 55-250mm
70-200mm F4 IS(around $1100)- many call it the sharpest zoom lens that Canon has, fastest of three noted with focusing, can accept a teleconverter(stay with the 1.4x if you go this way) and has a constant F4 aperture.
Wide angle:
The two best in that area appear to be the Canon 10-22mm($690) and the Tokina 11-16mm($570).
Flash:
Most people are fine with the cheaper 430EX flash, which you might wait on because their coming out with the 430EX II soon. This should run you about $280.
I have had the Canon 40D for lose to a year and love it. I would like to buy a telephoto lens, a wide angle lens and an external flash. Does anyone have any words of advise on what I should buy?

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