Alternate Frame Rendering is preferred these days over Split Frame Rendering as the cards don't have to calculate frame geometry before deciding how to split the workload but I'm not sure what the difference is between AFR1 and AFR2, nor how AHII responds to SLI. My guess is that one of your AFR options improves picture clarity (anti-aliasing... probably AFR2) while the other increases frame rate (probably AFR1).
[EDIT]
Here... I cut this out of an article at Wikipedia for you:
SLI offers two rendering and one anti-aliasing method for splitting the work between the video cards:
Split Frame Rendering (SFR), the first rendering method. This analyzes the rendered image in order to split the workload 50/50 between the two GPUs. To do this, the frame is split horizontally in varying ratios depending on geometry. For example, in a scene where the top half of the frame is mostly empty sky, the dividing line will lower, balancing geometry workload between the two GPUs. This method does not scale geometry or work as well as AFR, however.
Alternate Frame Rendering (AFR), the second rendering method. Here, each GPU renders entire frames in sequence – one GPU processes even frames, and the second processes odd frames, one after the other. When the slave card finishes work on a frame (or part of a frame) the results are sent via the SLI bridge to the master card, which then outputs the completed frames. Ideally, this would result in the rendering time being cut in half, and thus performance from the video cards would double. In their advertising, Nvidia claims up to 1.9x the performance of one card with the dual-card setup.
SLI Antialiasing. This is a standalone rendering mode that offers up to double the antialiasing performance by splitting the antialiasing workload between the two graphics cards, offering superior image quality. One GPU performs an antialiasing pattern which is slightly offset to the usual pattern (for example, slightly up and to the right), and the second GPU uses a pattern offset by an equal amount in the opposite direction (down and to the left). Compositing both the results gives higher image quality than is normally possible. This mode is not intended for higher frame rates, and can actually lower performance, but is instead intended for games which are not GPU-bound, offering a clearer image in place of better performance. When enabled, SLI Antialiasing offers advanced antialiasing options: SLI 8X, SLI 16X, and SLI 32x (8800-series only). A Quad SLI system is capable of up to SLI 64X antialiasing.
Nvidia has created a set of custom video game profiles in cooperation with video game publishers that will automatically enable SLI in the mode that gives the largest performance boost. It is also possible to create custom game profiles or modify pre-defined profiles using their Coolbits software.
For more information on SLI-optimized games, visit Nvidia's SLI Zone.
BTW, you could do us all a big favor in the ongoing SLI debate here on the BBS by running AH (or another game) with a single card, then again with both cards in SLI and reporting back on your FR's and experience. If you're comfortable enough to get really technical, you could D/L the free version of 3DMark and bench it in both configurations.
