Hi Andrew ...
Safari does the same thing for me in TopSites on my Mac running Snow Leopard v10.6.8. I've yet to have any issues with the computer as a result of it. The anomaly doesn't show up in Safari on v10.7 Lion.
You can provide feedback to Apple > Apple - Safari - Feedback
Just for peace of mind, you can run the Apple Hardware Test just to rule out any hardware issues.
I have had this problem for a while now. The problem ONLY occurs with Safari if I open a new tab and have preferences set to top sites. What I see when I open a new tab is pixelization and/or flickering. I thought it was hardware issue so I called Apple. We ran HW tests and they told me it was not. They advised it was likely a plugin or software issue. We re-installed Snow Leopard (maintaing all data) and that did not help. We did tons of other things, all to no avail. Apple suggested I do a WIPE and start over. They also suggested I do NOT reimage from my time machine backup as to not bring back whatever plugin or SW
So I finally got around to that this weekend. I am on an essentially virgin system. The only two apps I installed are Handbrake and Skype. I have not done any flash update, plugins or anything else. The problem still exists
I have an excellent and short video capture that shows this but not sure how I can upload that here.
I guess from reading this post that I should go to Lion - which is on my list of things to do but need to check on compatibility of some SW/HW first.
OH btw, some specifics: Imac 27" Snow Leopard, 12GB Ram, all SW is up to date
Just thought I would throw this out there...