yes, wait to update until 4.1 comes out.
If the only problem is slowness, I have been told by the Apple Genius Bar to turn off spotlight search and it will act normally, I have also seen this posted on forums, so any 3G user that updated should try this first.
My 3G lost ability to connect to wifi, so be patient and don't update! I ended up getting a new 3G iphone from Apple but lost some data in the process of trying to fix it before I got to the AppleStore. (I had no problems with my iPhone 3G for 1 1/2 yrs--until I updated to iOS 4!)