I'm running CS3, and after I installed the Adobe Plug-ins, I couldn't get AfterEffects to play back rendered RAM previews in real time. I had to uninstall the Adobe Plug-ins to get AE working again. You should be able to look at RAM previews over your AJA output without the plug-ins anyway.
You can. If you go into the AJA Preview setup there is a check box to turn off the preview. Just un check it and you will be back to just the AE Ram player. To make your preview performance better make sure auto scale is unchecked and your composition settings and playback settings are the same. Depending on your system that should get you closer.