I know cannot use them to SLI... but perhaps can i use my 260 card to run ONLY for the physx it has and use the new card for everything else? or/and use the old card to only run one monitor and use the new card only for my primary display? Will i get improvements for doing this or will it be negative?
Nope dont waste your money on something like that especially since a lot of games dont even support physx.
You're best bet would be to sell the 260 gtx for ~$100 and get a 560 ti or a 6950 for around ~$200 which will give you much more performance than a 550
The clock speed matters much less than the actual core count in a gpu, and the 260 also has a much larger memory bandwidth. All in all it's only slightly more powerful than the 260. The 550ti is really not a very good option at its price point, I wouldn't recommend it to anybody, you would be better of goign with a gtx 460 or like bavman said sell the 260 and get something like the 560ti or 6950 in the 200$ price range.