You'll want a second slot regardless, octane uses 100% of the GPU, meaning even using the menus and stuff IN octane are painful without a display card.
You can go with either 2 render cards like the GTX560 and just set it to use only one (the one not plugged into your monitor) or you can get a super cheapo card like an old 9800GT or something just to run the OS and display output while octane runs on the GTX560.
Here's a quick chart I made for cost to value. I used the CUDA Folding @ Home benchmark for the score, then divided the price by the score to get a cost per point. Then for perspective added the costs for having 2 or 3 of each card and that cost, since you ARE limited to the number of cards you can fit in your box!
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card: score: price cost/score 2x score 2x cost 3x score 3x cost
GTX 580 379.7 $500 1.32 759.4 $1000 1137 $1500
GTX 570 342.1 $350 1.02 648.2 $700 1026 $1050
GTX 560 ti 292.8 $250 0.85 585.6 $500 876 $750
GTX 480 329.1 $399 1.21 658.2 $800 987 $1197
GTX 460 218.7 $169 0.77 437.4 $400 654 $507
Though I'm not 100% sure how well the CUDA benchmark score maps across to ms/sec in octane... I would assume it would be near the same curve?