The OP's problem is not universal, my Safari 5 on a MacBook 1.83 running 10.6.4 with 3 gigs of ram does not exhibit this behavior, nor does it on my G5 iMac running 10.5.8 (iirc) with 1 gig of ram. It also runs fine on a couple of mini's and a Hybrid machine here in the house.
So -
Try making a new user account and see if you have the same problem. If it works correctly in the new account, there is something in your user account that is screwing things up. If it doesn't work in the new account either, the system has an issue.
If it works, you could go to your regular account and remove the safari folder from the Library folder in the user account (not the system Library). Also remove com.apple.safari.plist as well. It is located in the Preferences folder in the user Library. Just put them in a folder on the desktop for now. You can replace them if this doesn't work.
Then boot-up Safari. It will regenerate all those files. If it works now, just trash the preferences you removed.
This is a bit of overkill. It may be just one of the files you trashed, but i don't know which one and since they regenerate, no harm no foul.
If it doesn't work in the new account, I do not know how to advise other than Couriosities remedy.