First off, do you have sufficient power and cooling? Because you're building something serious here.
Furthermore, I wonder what the rest of your system looks like. Since it might be that your third card, regardless of the physics, wouldn't add more performance, because it's not the bottleneck. So please update your post!
From a review by Guru3D I'd say that getting a third GTX 280 would be the best improvement:
Alright, the conclusion really doesn't need to be very long. Money wise high-end SLI and 3-way SLI doesn't make much sense, and it never will. It's bragging rights, and only for very few of you with a 30" monitor actually a wish. See, the GeForce GTX 280 by itself already is a power house.
So as much as I'd like to validate the existence of setups like this, common sense always kicks back in and says: dude .. performance versus money wise you better do something else with your money. And that's the cold harsh truth. This setup as presented today doesn't make much sense when you relate it to the core cash you need to drop.
But hey .. then there are a couple of Guru's out there who will go for these bragging rights, who do want to play their games at the most extreme resolutions with the best AA modes, and these kind of consumers .. might find this a valid purchase.
Now I'm a hardware freak, so certainly .. I won't hold a grudge against you man. In fact, you get my two thumbs up. Because the fact remains that when you factor the money out, the SLI setup as tested today is just an awful frickin lot of fun. I mean playing Call of Duty 4 at 2560x1600 with 4xAA and every possible image quality setting set to the highest possible setting returns an average framerate of nearing 80 with SLI; that will make you laugh a little giggly. Once firing 3-way SLI and to play the game at the same setting at near 110 FPS .. you'll giggle again, yet in a weird girlish gay kind of way, it surely is fun folks. No doubt about it.
So, allow me again to revert to something I mentioned in the introduction. GeForce GTX 280 SLI is much like Pamela Anderson's SLI boobs .. grand, maybe too big, certainly a nice pair to look at .. but surely very expensive. The same goes for Triple SLI which honestly doesn't make sense money wise, but yeah .. it does scale well and we were finally able to Play Crysis in Very high detail, which is just sick to see ... it's that much fun.
Upgrading it with a 9800 would make your system as good as it's weakest link: so either buy a third GTX 280 or don't buy anything. But you also have to consider if it's actually worth it? Since the performance gain you're aiming for will only be noticable at very high resolutions.