Andrei,
To answer your first question, the Polygon Count, when set low, will result in lower-resolution images, but once you get to about 40 or so (about half the maximum), adding more polygons will generally add very little visual enhancement. In addition, the Polygon Count is actually one of the parameters that puts the most stress on the GPU.
I mention the stress on the GPU because this is most likely why the text is disappearing when you set the Anti-Aliasing to best. To check this, there are a few things you can do to reduce the stress on the GPU. First, try creating a 3D title of nothing but a single letter, and set the Polygon count low (maybe to around 10~15). Also, dis-engage any lights or particles if any are in use. Lastly, if you are using a multi-monitor setup, try unplugging/dis-engaging the additional monitors. If you can reduce the current usage of the GPU, then you might be able to get the Anti-Aliasing, set to Best, to display the Extruded Text.
What kind of Graphics card are you using? Also, what is your host, and which version of BCC do you have installed?
Thanks,
Jon