I've seen some threads that people got 10.6.8 working for them, and it looked like they went through hell to make it work. That isn't worth it to me to maybe not have sound or wifi, and whatever other problems they were saying.
If you want to try it, there are some extensive threads around this forum, and I think the "OS X for Dell Mini 9" folder also... If you're going to try it, I'd do it before you add any applications/pics/music etc. to your 10.6.7 setup because if it doesn't work, you're probably going to have to reinstall 10.6 fresh and go to 10.6.7 again. Not saying don't do it, just beware!
Edit--also I'm not sure that 10.6.8 was a very essential update...you'll have to check!