I agree - buying a camera can be as frustrating as buying a computer. As soon as you get comfortable with it, the next model makes it obsolete and that happens very fast. But we must be hardened to that reality and only buy when we really think it worthwhile in terms of the bird in hand.
I only upgrade my computer when I find it really worthwhile. My first computer took a whole 2 minutes just to open an 18mb PhotoCD file. That was intolerably slow and there was great need to upgrade. I've been using my current Win 64 bit system with 6GB of RAM and a WD Raptor 10,000 rpm hard drive solely as a scratch disc, for over 3 1/2 years now and feel no urge to upgrade because it's still a reasonably fast computer and gets the job done.
Now, on that note, as the birds sing, the 5DMk2 has 36% larger pixel pitch than the 50D and it's low light ISO is 2.1x that of the 50D. Along with the higher resolution those must be reasons why you should expect better image quality from it.
Let's have a look at some hard facts from the DXOmark website. The 5D2's SNR, DR etc are approximately a whole stop better than those of the 50D. For example, at ISO 3200 the 5D2's DR is 9.09 EV, the 50D's DR at ISO 1600 is 9.19 EV.
A similar situation applies comparing the 50D at ISO 400 with the 5D2 at ISO 800, and the 50D at ISO 800 with the 5D2 at ISO 1600, with regard to all the parameters of noise, DR, tonality and color sensitivity.
However, oddly enough, between ISO range 100 to 400, the DR of the 5D2 is only marginally better than that of the 50D, at both the pixel level and the normalised print size; that is, less than a stop better.
Now such an improvement certainly appears to be worthwhile. However, when one brings into the equation the sort of lenses one might use with each camera, that apparent advantage might sometimes be non-existent.
For example, the lens I would use most with a 5D2 would be my Canon 24-105/F4 IS. For very wide angle requirements I would use the D700 with Nikkor 14-24/F2.8. For long telephoto work, I would prefer the 50D with my 100-400 zoom.
But supposing I compare the 50D and 17-55/F2.8 combination with the 5D2 and 24-105mm? The EF-S 17-55 doesn't have quite the same range as the 24-105, being only 27-88mm full frame equivalent, but it's reasonably close. Whatever aperture I choose for the 5D2/24-105, I can choose one aperture wider
using the 50D/17-55. The EF-S 17-55 is F2.8 across the whole FL range, and the 24-105 is F4 maximum across the whole FL range.
If I want extensive DoF and decide to use F11 with the 5D2/24-105 at ISO 400, I can use F8 or even F6.3 with the 50D/17-55 at ISO 125-200. If I'm shooting in the street at night without flash and need to use the 5D2/24-105 at F4 and ISO 3200, I can use the EF-S 17-55 at F2.8 and ISO 1600 with the 50D.
In this situation, I can't see any advantage for the 5D2. With another combination, comparing the 5D2 and 85/1.2 with the 50D and 50/1.4, for example, I can see there would be an advantage in terms of sharper and cleaner images with a shallower DoF, but not necessarily at smaller apertures. I think the 50D with 50/1.4 at F6.3 would be very close to the 5D2 with 80/1.2 at F11, bearing in mind that the resolution advantage of the 5D2 sensor is a mere 18% greater than the 50D.
Have I just talked myself out of buying a 5D2?