IE8 starts to die on my boxes once I have more than 10 instances open with maybe 6 tabs each or just a couple of instances with 15 or 20 tabs.
Admittedly my own machines are getting a bit long in the tooth but I even have trouble on my works relatively new Dell Latitude E6500 bought last year, that's running 64 bit Windows 7 with 4GB of RAM, same problem on my work PC that I use for development.
I'll need to try opening lots of IE8 tabs on my T61, it's an older laptop but it has 64 bit Win 7 and 8GB of RAM, maybe it's just the RAM and swapping but it feels more like an old fashioned system resources issue like it's running out of handles somewhere.
Reason I'm using Chrome is purely for the speed of the Javascript interpreter, much much faster rendering than the current IE.