Radeon X1950Pro vs. Radeon HD 2600XT
conversion and editing have no major difference at this point, but for viewing the HD2600 offers more options and does it more efficiently than the X1900Pro.
The 128bit / 256bit question is more of one for larger resolutions and AA levels, where there's a slight bandwidth benifit to the X1950Pro, however the main performance issue there won't be bandwidth it'll be the way the HD2K series handles AA, and at higher resolution with AA enabled the X1K should be much better.
ithout AA and especially in newer titles the HD2600 may pull ahead in shader intensive situations, but I would say a good rule of thumb is if you're a gamer and plan on refreshing again in a year or so, then get the X1950Pro, but if your focus is more on the HD playback, and you might not upgrade for 2 years or so, then probbaly the HD2600 will give you a better fit.
However, if you're willing to wait a few days, a cheap HD3850 may be the better solution, with faster gaming power than both, and with all the nice HD playback benifits of the HD2600XT (with the potential of even better post processing optimizations, and even the potential better GPGPU situation if they ever get around to using it for encoding, etc).