I have bought piece by piece basic used Canon equipment (70-300mm 1:4-5.6 IS; 28-135mm 1:3.5-5.6 IS; 50mm 1:1.8II; mechanical macro extension tubes set; and a 40D body) because I haven't been able to afford high end stuff.  My choices have been made based on my understanding of info gleaned in this forum. I've finally decided to get one nice lens. My criteria is simple.  What is the lowest cost lens that will produce "Eye Popping Images" compared to what I have?  I figure I'm going to have to buy a Prime lens and an extender.  I like bird shots (maybe 50 feet away) but I'll learn to take pictures of subject matter that fits within the focal range of the best cheapest lens I can afford and live with what I have.  I believe if you can't affort a yacht then find a boat that does something exceptionally well within your purse and work around that!

One more point! I've achieved many nice shots and a few eye popping pics with the 70-300 but not many.  Maybe I'm relying to much on the "P" mode and I need to learn more about setting the fstop manually.  So if you recommend gaining more knowledge rather than buying a better lens right now I will understand and appreciate your comments.  This is my first digital camera and maybe I'm expecting too much from the lens I have and not enough from me!

P.S. I'm new to registering on this forum but have religiously read the posts over the last year and a half. I've really appreciated not only Bob Atkins valuable information but also everyone else who poses a question or inputs.  It takes both groups (expert and not so expert) for me to understand sometimes.

Thanks for any comments.

This might help:
http://www.kenrockwell.com/tech/unsharp.htm

Excellent responses by everyone and very helpful. Thanks!

"I've achieved many nice shots and a few eye popping pics with the 70-300 but not many.  Maybe I'm relying to much on the "P" mode and I need to learn more about setting the f stop manually."

When using "P" mode you don't have any control over lens opening. So, before spending more you might want to set your camera to use the sweet spot of your lenses. For example, set the 70-300 to aperture priority and f8. I had this lens and found it to be extremely sharp at that setting up to 250mm.

Of course you also would need to eliminate camera shake to get the best out of your lens. Only after you do this will you be in a position to understand whether or not to purchase other lenses. My guess is that your percentage of sharp pictures will rise dramatically by taking this approach.

EF 50mm f/1.8 II for $99 is certainly cheap (if not the cheapest!) and many people say it is sharp as a razor.   The only problem is that quite a few people say it breaks easily.

I know, Bob, and it is funny to see some reviewers giving it a 1-star rating on Amazon because they dropped theirs...  however, a bigger risk seems to be this lens getting stuck on a camera body forever, especially on XSi...