A couple ways to look at it. If I were you I wouldn't invest a lot in my current system and I would wait for the affordable 6 core when it comes out. I would consider some changes to your current rig.
1. BUY AN AFTERMARKET COOLER AND OVERCLOCK
Buy an aftermarket heatsink for your Q9300 (see this link for reviews
http://www.frostytech.com/top5heatsinks.cfm but make sure it matches your socket 775 and don't spend over $80) and overclock it. This review manages to play crysis with an oc q9300 @3.4ghz and an 8800gt at respectable frames per second up to x1200 resolution then it gets choppy. http://www.overclockersclub.com/reviews/intel_q9300/
2. WAIT FOR GTX 480/470 IN APRIL
Buying the top of the line 5970 video card or anything over $300 seems short sighted considering we are so close to getting benchmarks on the GTX 480 & 470 cards in April. They could be superior in every way and you may be kicking yourself in just 2 months if you buy the best ATI now. Anyway based on Tom's charts the 9800gx2 is a decent card and shouldn't require replacing just yet. You could overclock it a small margin just to gain some more performance while waiting to see what the GTX 480/470 look like in benchmarks and pricing. Buying the best video card soon wouldn't be a mistake if you have the money and see the value in it, you can always transfer it to your future upgraded setup and not worry about buying a new video card at the same time as everything else. Subjectivity is all over when describing what video card is reasonable or not. It depends on so many variables... resolution, type of game, quality of monitor, preference of gamer, high fps and lesser graphics.
3. START TRAINING YOURSELF IN OVERCLOCK NOW
If you take this advice and wait for the 32nm 6 core you will have the hardware with potential to overclock to great limits (i hope). So now is the time to get familiar with overclocking so you are ready to do so proficiently when you get that 32nm chip. Regardless if the 32nm turns out to be great you at least gain performance now at little extra cost.
To answer your questions:
Your current graphics card is an appropriate fit with your current setup without overclocking.
Yes a top o the line ATI like the 5970 would be leaving processing power on the table with your current cpu (depending on the application).
Just so you know Im not waiting for the 6core, Ive been waiting long enough. See my sig I have a dual core amd 3800x2 (2.0ghz pre overclock). Im getting the 930 tomorrow. I probably won't upgrade for another 4 years or more. Just bad timing for me not falling in with the 6core but that will always be the case.