I'm a pretty big fan of the x3460 as a great bang-for-the-buck processor option. Usually it makes more sense just to get two servers each with one of these, rather than getting a larger server.
That said, I'm having a hard time finding real world performance data on faster cpus, as some of my customers could benefit from a faster single server vs using more than one. I see that the 6 core Xeon E5645 (2.4ghz) is only costing about $600 apiece these days. Considering you could get two of these into a system, on paper it would sound like you can get about 29ghz of processing power for about $1600 (motherboard and cpu costs only), vs 11ghz for the x3460 @ around $550 (again, cpu and motherboard only). If you could take the mhz at face value, then it would be a pretty solid option to get the most processing power per dollar, when you consider all the other costs in running a server.
The part I don't know about, is how the E5645 really performs? The cpubenchmark.net results are really only showing for different cpu models than this, and I'm not sure if the performance on this one scales perfectly with clock speed compared to the other model numbers that are ranked. Also, some of the results on cpubenchmark are a bit wonky. Like they have the xeon E5345 at around 2800 marks, vs the x3460 at over 6000 marks, whereas in my usage, a dual E5345 is faster than a single x3460 by a noticeable margin. Although the cpubenchmark.net results are probably good enough to figure out to within 20-40% of a cpu's speed, I really would be looking for something more accurate than this.
Any idea of the real world performance of some of these options? x3460 vs dual 5620 vs dual 5645? Mostly php and mysql type performance in mind.