iMac has had a sudden decrease in wifi download speed

Try this to check you router:

http://netalyzr.icsi.berkeley.edu/

You will need to enable Java for it to work.

It will check your DNS, internet speed and the router (To see if it is working properly and for buffer blot.) It also checks a few other things.

Those internet speed checkers are really useless.

I don't think you can change the buffer in the router. Here is a video that explains it all and buffer blot:

Security Now Show that is on the TWIT (This Week In Tech) Network.

http://www.podbean.com/home/podcast-directory-play.php?eid=4923969

My recommendation would be to change your DNS settings, if you are using the defaults those are supplied by your ISP and could be your problem. If you are not familiar with DNS settings please read CNET's DNS Article and MacWorld's DNS Troubleshooting and then change them to:

1. System preferences

2. Network

3. Click Advanced

4. Click DNS tab

5. Click + sign

6. Add 208.67.222.222

7. Click + sign

8. Add 208.67.220.220

9. Click OK.

10. Exit System Preferences

It's possible the router is failing or needs to be reset. Exactly what router make and model is it?

Are you still running Snow Leopard 10.6.6 as your profile states?

Have you done some router testing, for example have you reset (including a hard reset), checked that it has the most up-to-date firmware? Also exactly what router make and model is it?

Do you live in a suburban area, if so you also might be fighting for channel bandwidth with your neighbors. Download and run iStumbler (Google it) and check to see what channels you and your neighbors are using, most routers default to channel's 5 or 6 so if you and your neighbors are all using the same channel then all you need to to do is switch to another channel. Let iStumbler run for about 20-30 minutes to get a good overview of the area.