I am building a computer in the next week or two and am having issues deciding between the GTX 275 and GTX 280. I am buying from Newegg and am going to purchase one card for the time being and later purchase another for SLI. The issue I am having is the GTX 280 is older (currently only 2 models available for purchase on Newegg) and the GTX 275 is newer (at least 15 different models available on Newegg) and am worried that if I go with the GTX 280 I will not be able to find another down the road when the price drops happen again (maybe 6 months or so later). Which would you purchase?

ALSO: Is it worth buying a better graphics card now considering the LAUNCH of DX11 later in the year? Would I be affected and disappointed with the purchase a year down the road?

I am open to any other suggestions as well regarding the graphics cards if you believe there is a better solution. This will be a gaming computer and gaming performance is my primary concern for this build.

SYSTEM THE CARD(S) WILL BE PAIRED WITH:

Intel Core i7 920
6GB GSKILL DDR3 1600 8-8-8-21
Corsair 850 Watt PSU
Coolermaster HAF 932 Full-Tower Case
HAVEN'T DECIDED ON A MOTHERBOARD YET BUT IT WILL HAVE SLI CAPABILITIES (ANY SUGGESTIONS FOR A MOTHERBOARD - MAX $250)

Was looking at this motherboard: http://www.newegg.com/Product/Product.aspx?Item=N82E16813188049

My only concern with this motherboard is it doesn't seem to run 2 cards at x16. I believe I need a motherboard that is capable of running 2 cards at x16 right?

Thanks in advance.I would purchase the GTX 275. Performance is very similar, sometimes matching or beating, the GTX 280.
By the time you "need" SLI to run games, I would guess that neither of those cards will be produced any more, but you'd easily find them for sale on Ebay or various forum trading sections.
In my experience very few people upgrade to SLI when they buy a single card to begin with, most go and purchase a new, more powerful card instead. This avoids the sometimes HUGE power requirements of SLI and avoids the issues with multi-card solutions.
You shouldn't worry about x16/x16 SLI capabilities, cards don't tend to COME close to exceeding the bandwidth requirements for PCI-E X16 (version 1, PCI-E 2.0 bandwidth is double so X8 = gen1 X16 speed, if that makes sense. New boards are PCI-E 2). This board will run one card at X16 and the other at X8 speed, which is plenty fast enough.
Most of the Core i7 boards out there are great, so the board choice isn't too important ... they're practically all high end boards, with matching features.I guess a GTX 280 because you will get more FPS in games.Thanks Calum for the advice, it really helped. Should the release of DX11 worry me at all when thinking about purchasing a graphics card now? I have heard most games won't begin using DX11 for quite some time (maybe a year or more from now).I wouldn't base your purchase on DX11 support.
Think back to 2006, when the first DX10 cards were released. By the time DX10 games were released, the cards couldn't handle the games at that level of detail (think 8800GTS and Crysis on maximum - Crysis under Vista runs in DX10 mode, but you won't see any difference unless you go to the maximum settings, which is the only level where DX10 effects are used I believe).
It'll most likely be the same with DX11, by the time any decent games that use DX11 in a way that makes you want to use it are released you'll need a new card anyway. Besides, you can always play them in DX10 mode, so I don't see that being a problem.
I certainly would just go for whatever is the best value and not worry about features that may or may not be necessary or desirable in a few YEARS time.