GeForce GTX 280 or GeForce 9800 GTX+ SLI

9800 GTX+ sli is just a waste, although they are faster than the 280 GTX, your either better off getting a 280 GTX, 260 GTX or settling some where in between for a 9800 GX2.

Only reason I'm not suggesting an ATI card is because I got the assumption that you have an sli board.

Only other option would be to bite the bullet and grab an 4870 X2 

Well, two 9800 GTX+ cards in SLi would generally be faster than a 280GTX and slightly faster than a 9800GX2. Of course the 9800GX2 is going rather fast so supplies are limited as they say  . Anyway, compare charts to see the difference between the 9800GX2, which is basically two 9800 GTX cards in SLi, and the 280 to give you an idea of how two 9800GTX+ cards will fare in SLi. While in most cases the SLi cards would be faster, there are a few instances where SLi does not scale well and the 280 comes out on top. The 280 also comes out on top at high resolutions and settings where the memory is taxed. The extra memory on the 280 will also shine in future games while the 512MB on the 9800 GTX+ may be a limiting factor in the future.

You need to take all of that into consideration before you choose. I say that if you can afford it though get a 280 GTX or 260 now and then another one latter to run in SLi. If you really need the power of two 9800 GTX+ s now then buy those and run them in SLi.

What kind of motherboard and PSU and case do you have?
What kind of video card do you have now?
Are you willing to consider ATI cards too or just nVidia?
What is your budget?
What is your screen resolution?
What are your favorite games?
What country are you in?

Aevm would also like to charge 10% for help 

Oh, yeah, I forgot: what's your credit card number?   J/k, don't type it.

Now seriously, if you want an intelligent answer you need to give us some details. For example we often find great sales at Newegg and recommend them and then it turns out the OP is not in the USA and can't use them.

Mobo: EVGA 780i
PSU: COOLMAX CUQ-1200B 1200W
Video Card: EVGA GeForce 8800 GTS OC 512
Budget: around 500
Resolution: 1920x1200
Fav Games: Cod4, Crysis, Half-life 2, UT3. This upgrade is is for Far Cry 2 and Fallout 3
Country: USA

Maybe ATI. i just like the Limited Lifetime Warranties that come with most Nvidia cards Plus EVGA has the step up program.

Impressive setup.

Considering your 780i motherboard, I'd say we stick to nVidia solutions. With ATI the best you can hope for is a single HD 4870X2, and then the expensive PSU and motherboard are wasted. BTW, your PSU can handle even three GTX 280 cards, according to this list:
http://www.slizone.com/object/slizone_build_psu.html

The cheapest and simplest possible upgrade is to buy another 8800GTS 512MB. The eVGA is no longer available at newegg, but there's a BFG for $150.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814143119
Unfortunately that card doesn't support Triple SLI so you can't buy 2 more and add them. Too bad because the PSU and MB could have supported that.

You could sell the 8800GTS and buy three 9800GTX+ cards. For example there's an XFX for $160. Not sure if they send 3 rebates to the same address, but you can always ask two friends to order one each for your Christmas present  
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150314

A more powerful solution: buy two of these GTX 260 cards ($263 each, but again no idea if they send more than one rebate). Sell the 8800GTS. Buy a third GTX 260 when you have more cash, and if you really feel the need for more graphics power.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130372


Edit: I think the idea with the GTX 260 cards is better. That is, games will support two GPUs better than three, normally. Plus with the GTX 260 version you still have a PCI-E slot available for future expansion. Also, the extra RAM on the GTX 260 will help at your high res.