I'm not sure if this is the best place for this post, but here goes:
Skyrim is coming out. Beyond that, I have Civilization 5, Shogun 2, and plenty of other games that love the idea of DX11.

I want an Nvidia card. Logic or not, I have a certain amount of brand loyalty and an unusual affinity for the Nvidia cards. http://www.nvidia.com/object/graphics_cards_buy_now.html

My current graphics set up is Dual GTS-250s in an SLI arrangement, sent to me by Alienware a few years back. I love my rig, but I would really like to upgrade the graphics capabilities. I'm looking at the http://www.nvidia.com/object/buy_now_results_ci.html?id=GFGTX550TI It seems to be the most powerful card in my price range (at or just above $100). What advice do the experienced users here have for me, if any? Is this card going to be more powerful than the dual set-up I have now? Are there other cards (for just over $100) that I should consider that might be more effective?

Please let me know what I can do to be most helpful to you all, and let me know if you have any thoughts, ideas, or experiences. Thank you so much web community!

I saw you said you wanted to play other games better as well, so this may not apply, but I'm in a similar situation to yours (I have a GTX 260) and I'm basically putting aside $10 or &15 every week until it comes out. Then I'm just going to buy the best I can afford a week before Skyrim comes out.

With 2x250's you will definitely be able to run Skyrim on full settings but only at 1680x1050, rather than the 1900x1200 res the 550 can pull.
If you value that small increase in res then go for it, if not then stick with your current rig I say.