Yeah, that's exactly what they use for step-up basis. Another thing is they only offer the stock clocked variant typically, so that's another thing to consider for people who prefer factory overclocked models. The GTX 275 they offer should be $250 though, you might've been looking at the 1792MB RAM variant.
It has its flaws and shortcomings, but there's been some cases where its been really good value for some people, although it typically only comes into play if you got fleeced the first time around (9800GX2 comes to mind). A few other recent examples were people able to step-up from 65nm to 55nm parts for free around January and GTX 260 to 275 for almost free.
i5 and P55 I believe are expected sometime late Q2, maybe early Q3 in time for back to school. Perhaps July if they try and hit the college kids. I know they had some boards at CeBit but Intel has pushed back the launch a few times due to high inventory of LGA775 and low global demand.
I was personally going to wait for i5 as well, as I'm not that interested in SLI/CF, but the price gap has come down significantly now to the point the only real difference in price is the motherboard. If you don't care about SLI, there's quite a few boards that are $170-180 and some can be flashed to SLI support. Be careful though, some of these lower end boards only have 4 dimm slots. There's still some premium on DDR3, but its about $10-15GB which isn't far off what DDR2 was not too long ago. Both LGA1176 and LGA1366 are said to be supported by Intel for at least this Tick and Tock (32nm Westmere)
Hard to say, SLI/CF solutions nowadays are so powerful they're already pushing the limits of the fastest CPUs, so they may continue to scale in performance as platforms and CPUs are upgraded when the new GPUs hit the market. A fast single-GPU would also be limited similarly, but should scale more efficiently of course.
The first DX11 parts are expected in Q4 this year with GT300 and RV8x0, and although I don't expect them to quite be faster than current SLI or CF solutions, they'll most likely be close without any of the hassle or headaches. Personally I wouldn't wait for that though if you're already gaming on an aging rig and have the itch to play modern games. I'd go ahead and upgrade, grab one of those GTX 260s for $170 (or see what a GTX 275 costs when you're ready to build), then upgrade again in December to a GT300 or RV8x0 if you think its worth it.