I'm playing FreeSpace 2 for the first time and I'm liking it so far. But with this system of mine that is many generations beyond FS2's time (Geforce 8800gts, intel e6600, etc) I seem to be getting really bad performance on missions that take place in the cloudy Nebula area (even when I throw all of the graphic settings down to LOW). It will start off pretty smooth but then it will drop significantly when there are 2 or 3 ships in my view. Sometimes in intense battles, it dips below 10 fps!
I don't have performance problems on clearer maps, though.
I'm using 185.85 nvidia drivers.

It might be a multi core cpu issue too. I ran it on an Intel E8400 (2 cores) and an nvidia 8800gt sli setup with no problems I never looked to see what my frames where but I never noticed any slowness either.

Once again, FreeSpace Open would probably improve the situation.

You are right.. FS open not only solved the performance problem, I can run it in 1280 x 1024 now, too.
However, I used the FSO Installer and did a FULL installation (took 2-3 hours) without really thinking about what I was doing and now I hear this annoying text-to-speech whenever there are messages/briefs that don't normally have a voice-over and its really annoying! Which mod is it, and how do I get rid of it?!

What build are you using? I've gotten the impression that 3.6.10 builds do not even have simulated speech. In any case, this is not caused by a mod; it's a feature. Open the Launcher, open the Speech tab and tick off all the options.
Also, if you used Turey's Installer (which I personally dislike) and if you selected to download the MediaVPs, you probably have corrupt MediaVPs. Downloading the MediaVPs manually is a recommended course of action.
One more thing: if you have any problems with FSO, you may want to visit Hard Light Productions. I've written an excellent Troubleshooting FAQ on HLP's Support board (pardon me for bragging), which covers all the most common issues that can be encountered with FSO. And if it doesn't help, you're always welcome to ask any questions you may have.