I have a ATI Radeon 9800 Pro and at 800x600 with all of the graphics turned up, I get smooth frame rates. Since the X800 Pro is at least 50% faster than the 9800Pro, you should be fine. Don't get me wrong, Doom 3's frame rates do drop on me from time to time but it is when I enter a new area. I don't believe this is because of the video card but because I only have 512MB of system memory and there is a lot of paging going on with my hard drive. When you notice the drop in frame rate, do you also notice your hard drive spinning? If so, the problem is probably paging. I would look to increase your system memory.
The performance jump you would get by going to the 6800GT would be minimal and definitely not worth the $400 (minus whatever you could sell the X800 Pro for). In addition, I bet that X800Pro plays Half-Life 2 faster than the 6800GT.
I have a feeling there is something else going on with your system when you get the low frame rates in Doom 3 other than your video card.
Oh...forgot to mention, if you did not upgrade your video card drivers lately or apply the Doom 3 patch, do so immediately.
I would get the sound system you desire and save the left over money for a future upgrade...don't spend it...save it...