Q: I upgraded my mac to 10.6.8 and it is much slower, especially downloading, do I need more RAM?  I have 2G

More RAM is always better. Check the specs for your unspecified iMac at http://mactracker.ca/ to determine the maximum your machine can use. Mine has 16 GB. Then, see:

Mac Maintenance Quick Assist,
Mac OS X speed FAQ,
Speeding up Macs,
How to Speed up Macs,
Macintosh OS X Routine Maintenance,
Mac troubleshooting: What to do when your computer is too slow,
Essential Mac Maintenance: Get set up,
Essential Mac Maintenance: Rev up your routines,
Maintaining OS X, 
Five Mac maintenance myths and
Myths of required versus not required maintenance for Mac OS X for information.

In addtion to all the links provided above, you can try the following site. Go to their how-to section, then read the section on using Activity Monitor to isolate performance problems:

http://www.scsc-online.com

The article is about Mountain Lion but I think it's nearly 100% compatible with SL, except some of the processes on ML may be new or unkown to SL. Just read the article, you'll get the point. Using Activity Monitor, you'll be able to see how much free  memory you have. Normally, unless you're using some apps that use a lot of memory, 2G would be enough on SL, but on the other hand I don't know what you're running.

Also, how full is your disk? If the drive is near full (less then 10%-15% free space) you'll likely find your system struggling. Also, with a new install, SL will go into full Spotlight indexing mode that will make anything seem slow until it's done, including networking. If I recall correctly, that stuff is in the article I referenced.

Another thing you may need to do is makesure you've done all your updates. I think their were several related to Airport and networking....I just don't remember for sure.

You may also want to read and consider resetting the SMC described in the following link:

http://support.apple.com/kb/ht3964

That one explicitely states:

System Performance
The computer is running unusually slowly although it is not experiencing abnormally high CPU utilization.
Application icons may "bounce" in the Dock for an extended amount of time when launched.
Applications may not function correctly or may stop responding after being opened.

Another one to try, but I wouldn't think it would help you is an NVRAM reset as follows:

http://support.apple.com/kb/ht1379

I really wouldn't think that would help, but the SMC reset might. Activity Monitor can tell you how much RAM your using.