The PERC 6/i is fully supported on the 1950 (although it may require the BIOS/ESM to be updated first), but you would need the Integrated version, as the Adapter version may not work properly in the 1950. If the drives are not compatible with the PERC 5 though (Are these non-certified enterprise drives or consumer/desktop drives?), they may not work on a PERC 6 either, unless the slight difference in the newer firmware gives the PERC 6 slightly broader compatibility with drives. The PERC 6 is still a 3Gbps controller ... the speed should be negotiated between devices and should really not be an issue - at least not the main source of incompatibility.
Rather than change my post above, let me just add to it (now after having just noticed your last line about certified server drives) ... no 6Gbps drives were certified for the PERC 5, and although there really should be no general incompatibility from speed, it is possible the newer firmware of the PERC 6's will properly handle the drives you have, as the systems sold with 6Gbps drives ALSO have a PERC 6 as an option - though the difference is small, it may be enough.