First off if you haven't purchased your 5800s yet, be sure to take a look at the price/performance of the new 6500 appliances. Based on SPU benchmarks a 6500 is about 20% faster than a 5800 but at about half the price. The 6500 is in a pretty nice spot. A 6800 is slightly more than twice as fast as a 5900 but for about the same price as a 5900.
Based on the blades you have enabled, SMT/Hyperthreading will probably be helpful. Run fwaccel stats -s on your current 4800, the more traffic you have in PXL (and to a somewhat lesser degree F2F) the more SMT will help. As far as the split for the new box that will once again depend on what fwaccel stats -s shows, but based on your blades for 8 cores I'd probably start with the default 2/6 and go from there.
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