In terms of graphical enhancements, you'll see a major difference in framerates in higher resolution games. Plus you'll be able to handle things like anti aliasing a little better along with better handling of great light effects. The improvement though isn't all that necessary from a 9800GT at a lower resolution game setting. Still, if you want a little bit more sharpness with the higher resolution, and a little less chugginess on explosions and stuff, you might want a new card. Plus, directx 11 is not capable on the 9800 GT.
There's two options for you for graphics cards. Get another 9800GT and run them in SLI or get a new card altogether and run it single in the PCI-express x16 slot. SLI is like crossfire, but for Nvidia cards. Crossfire is for ATI cards. Same concept. Two identical cards that combine efforts to get you better performance. Each installed in open PCI-xpress slots. Then connected together using a bridge of some sort.
Recommendations for the graphics card.
Identical or at least same number video card. Ex. 9800 GT made by any other brand. Best results from completely identical cards. And make sure to match the same amount of memory along with memory type like GDDR3 or GDDR4. It'll say in the specs. Clock speeds don't necessarily have to match, but the higher clocked card will clock down to the lowest for synchronization. From what I hear, Nvidia has better benefits from using combined cards compared to ATI.
If you want a new card, you might want to take a look at the gtx560. That'll run you around the $200 range. If you need something cheaper, go for the gtx460. I've got one and it seems to be doing just fine on most games that I have. That will run you around $140 or so. Not real numbers, just estimates. The Nvidia 400 series and above is directx11 capable. All other cards before the 400 series are not. So another 9800GT would not allow you to use DX11 stuff.
I would recommend the 560 if you really want to futureproof yourself at a reasonable cost, and 460 for better price. The performance boost you would get from a spare 9800 GT wouldn't be all that great compared to the power drain. You would probably have to buy a new power supply to support the power drain. The 460 is probably the best one of the 400 series, but the 400 series is still inferior to the 500 series, but probably not by too much. When the 460 came around, a lot of problems like heat, power drain, efficiency, and performance were addressed after the 480 and 470 had been complained on by bad managing of the same properties. The 500 series is improved on that along with a better tessellation processor to handle on the fly tessellation on a larger scale.
This tessellation implementation is a new feature of DX11. It allows for your 3d objects to exponentially grow in detail based upon the distance your camera is to your object.
I don't have any clue about ATI, but I'm definitely happier about having an Nvidia card. Nvidia cards have better drivers most of the time, and there's been a lot of development in using their multiprocessors for cpu related tasks. ATI is sort of the same there, but Nvidia has spent much more into encouraging and supporting that feature. Plus, you can still utilize your 9800GT to process hardware physx, Nvidia proprietary physics system, if you upgrade to a 460 or 560. Just stick the 9800GT in the spare slot and tell the system to look at that card for physx designation. Warning, you'll probably need a stronger power supply for that setup.
