Firstly, SLI is not what you are looking for. This term is not the use of two cards to drive four monitors but to drive 1 monitor at twice the performance.
I also think you are confused at how the clocks and video cards work in general. The 2D/3D clocks has nothing to do with 2D clocks can only accelerate 2D and vice versa. It is just a slower clock as 2D graphics do not require fast GPU, it is used to save power and lower heat in your computer.
The best cards for 2D used to be Matrox, but i am not sure anymore. My guess is the NV 8xxx series is just as good if not better. But then again what kind of 2D are you talking about? just normal applications like word, excel, and web browsers or are you talking about video performance like playing back video such as DVD, HD-DVD, and any HD video. If it is the first, normal GUI apps, then i say get any card you can afford from nvidia with 8xxx or ATI with HD2xxx series. If it is the second, video playback, then i would get the nv 86xx or 85xxx or ATI HD26xx or HD24xx as these have video hardware accelerators which help process the video data and some image quality algorithms.
Now with saying all this i wish you good luck and if i was you i would get the two of the same card for less driver problems, especially if you are only doing 2D stuff.