2 GTX 275s - = 1 GTX 295?

What resolution do u play at ?

Do you need to upgrade at this time? The first DX11 cards are not far off now, perhaps it would be better to wait until they are released and reappraise your situation then?

Check your PSU: Does it have enough PCI-E connectors? Does it have enough output? both of those configurations require a potent unit to feed them.
Cooling is also an issue with SLI/CF; are the PCI-E slots far enough apart and does your case have enough intake capacity?

And what resolution are/will you be using?

2 x GTX 275 > 1 x GTX 295
And 1 GTX 295 ~ 2 x GTX 260 216 Core

Go for 2x GTX 275.

+1
Absolutely correct.

The new GTX295 co-op PCB version that recently just came out comes close to two GTX275s though...but it's more expensive so two GTX275s are the better deal either way.

2 x GTX 275 > GTX295 co-op = GTX 295 > 2 x GTX 260 216

1 card is more than enough

I'm not worried about the PSU or PCI-E slots. However, I am intrigued about DX11. Is there really some good reason 2 wait 4 it? DX10 came out with lots of hype, but is has not impacted gaming significantly (I realize lots of games look lots better with it, but if u don't have it, it does normally make much difference). Will DX11 be anything special?

Here's the gist of it:
Microsoft unveiled Direct3D 11 at the Gamefest 08 event in Seattle, with the major scheduled features including GPGPU support, tessellation[11][12] support, and improved multi-threading support to assist video game developers in developing games that better utilize multi-core processors.[13] Direct3D 11 will run on Windows Vista, Windows 7, and all future Windows operating systems. Parts of the new API such as multi-threaded resource handling can be supported on Direct3D 9/10/10.1-class hardware. Hardware tessellation and Shader Model 5.0 will require Direct3D 11 supporting hardware.[14] Microsoft has since released the Direct3D 11 Technical Preview.[15] Direct3D 11 is a strict superset of Direct3D 10.1 - all hardware and API features of version 10.1 are retained, and new features are added only when necessary for exposing new functionality. Microsoft have stated that Direct3D 11 is scheduled to be released to manufacturing in July 2009,[16] with the retail release coming in October '09.[17]

Basically, it won't be a major major upgrade, but the graphical enhancements will be pretty big. Also, games will be techincally easier to run, as it will fully support multi-core processors. I wouldn't necessarily wait for it, but it will be nice. Just remember, the first DX11 games won't come out for about 6 months after the release. So April '10, expect to see some games.

Theres more to DX11 than that alone. Up to 3x performance using ambient occlusion, includes the DX10.1, having to use 1 less pass for AA, which is showing a 15-20% increase in fps. The 275s dont include DX10.1, and obviously the AO workable solution in HW either.
DX11 is an easy patch, so many DX10 games will be patched over to DX11, which will include alot of whats already been listed here, except tesselation, which does require more work

@L3v14th4n: Sorry, I meant the new DX11 hardware, not the software, according to the Internet Rumour Mill it will be faster and based on the smaller 40nm fabrication process, which should produce less heat and need less power.
DX11 will be, or should be, available to Vista users through an update but some of the extra features such as tesselation will require the new hardware.
If you wish to wait for the 'next best thing' that is fine, if you wish to upgrade now, that is also fine, it'll be a while before DX11 hits the mainstream but I would advise you wait, if you can.
Word is the ATI cards will be out on or before October this year with Nvidia following Q1 2010.

Here's a link to a nice table comparing the new ATI high end parts to current cards to give some insight into what may be around the corner
Though it is from late April:

Linky

I really like that bit about 1 less pass for AA, I had somehow missed that till now.

very good ^^

Just check out the benches on ATI cards on HAWX, DX10 vs DX10.1 using AA
Being that HAWX is a DX10.1 game and using a ATI DX10.1 HW compliant card, the 4890 beats out the 285 consistently on alot of reviews, if not all, other DX10.1 games show this boost as well, and any future DX10.1 or DX11 gane will show it also, only on DX10.1 compliant cards tho

You state this as though it's a foregone conclusion, do you have evidence to back this up?, or is it fairer to say that some games may be patched whilst many may not.

Well, splitting the difference between some and many? Ill stick to many. Realise, patching from DX9 was alot more intensive, and DX11 isnt, so, just by going on that alone, should give you your answer. I already posted 1 link awhiles back, they did it in 3 Hr time. Ive talked with other devs whove done alot in an evenings time, thats 1 dev, 1 partial night. Yes we will see it, but, the caveat is, as always, whats the benefits. If the games dont use alot of it, then no, but most do

Anyways... Going back to the OP's question, I would grab a 295.
I bought 2 and what a CF that turned into. One was the sandwich, the other was the single pcb. Boxes were identical, stocked in the same spot on the shelf. Got them home before I realized it and had the chance to try both out. The dual pcb sandwich was utter crap. I could have just got a bad one but the clipping and artifacting was more than I will tolerate out of a 500 dollar card. The single pcb ran flawless, ran cooler and conecptually better IMHO.
the reason I say 1 295 over 2 275's is you can put it on any board and still have the performance. You get 2 275's and you are painted into a corner. Either you get a nForce board or you go X58. the 295 could care less. the nf200 is on the card so it is SLI in one package.
Do yourself a favor and dont close your mind to the possibility of a ATI 4890. It is quite a beast at the price point.

I'm not worried about 1 card vs 2. The x58s are SLI/Crossfire-compatible, and it's reasonable to assume that most, if not all, high-end mobos made for high-end CPUs (i.e. i7) will support both of those configurations.
I would rather get two cards, mainly due to price. I'm on a tight budget, and $100 is something that can be put towards something else. (This doesn't mean I'm going to skimp on high-quality components, but I don't want to spend more than I have to.) Also, I will probably build another computer in 2-3 years, so i figure 2 275s will serve well enough until then.

As much as I buy from Nvidia and defend them, etc if you are dead set on 2 cards, I would seriously consider the 4890. Or....buy one 295 and then buy another later =)

Are there any possible compatibility issues? I will be running an i7-950 on an Asus P6T Deluxe mobo. I don't know any reason why there would be, but i just want to check.

Other than ATi's drivers being incompatible with their hardware, no.

P6T does not have a nF200 chip on it. plenty of people run CF on X58

What exactly is a nF200 chip, and how does it affect me?