CUDA cores - graphics cards nowadays achieve their speed in part by conducting multiple calculations at once, in parallel. The CUDA core (an NVIDIA term) is the base element of this parallelism, so the number of cores is essentially the number of simultaneous operations the card can undertake.
Texture fill rate is (roughly speaking) the number of pixels that the card can process a texture on per second. Generally speaking, the higher the fill rate, the higher the performance of the card, though other factors apply as well.
The note regarding two cards is in reference to SLI(scalable link interface). This is NVIDIA's technology for using two or more graphics cards to increase performance. You do not need two cards, but if you do want to use multiple cards, the note indicates that they both need to have the same processor (i.e. the GeForce 550 Ti).
Mike