Canon has a EF 28-135mm IS USM ( really nice lens for ~$500). The EFS 17-85 IS USM suppose to be just as good as the 28-135 for $600 but i'll find mine out wen i get mine in. EF70-300 USM (i want to say around $300). The IS version is around $500 but should be less then the 28-135 IS one.
In my opinion all of these take nice pictures and have very good build quality.
For Focal lense enhancers Canon has a 1.4 and a 2x, but i know another company makes them that are compatible w/ canon and are cheaper. Its one of the company's u stated.
The 550ex speedlite flash is nice runs around $400. Supports most of the features for the 20D.
The 580ex speedlite flash supports all features of 20D, runs around ~$480.
The 420ex speedlite is very nice for it's price running around ~$170. (a lil stronger power compared to off brand companys)
From what i used, the 550ex and the 420ex performs very nicely. For canon SLR i really don't suggest an off brand company since u loose alot of features. Well unless you are using just auto mode any off brand flash for canon slr should do.
Try
http://www.adorama.com , bhphotovideo.com , and ritzcamera.com.
I Hope this helps.
~Loveofselene