I thought turbo was only used when other cores weren't as busy. Could be wrong, but I don't think there's a difference between bottom 3. As far as crunching on all cores is concerned.

The i7 will allow you to run 8 processes (assuming you have HT on). Yes, turbo is basically meaningless when all cores are busy. Intel is going to make it appear best from a marketing perspective, but turbo only kicks in when there are idle (or nearly so) cores which allows the active core to speed up without overheating the chip.

As with any chip, a faster clock speed will give you more ppd. Mobile processors will give you a lower performance than desktop chips, but they also use less energy. Buy the best you can afford is the best advice anyone can give you to maximize ppd.

Experience opine: Not the 2720QM as that was succeeded by the 2760QM. As for the choice, look for a gaming laptop... more cooling design included, so that's what I got after letting several run with BOINC on it in a shop for 24 hours... and getting 3 years guarantee. 70 watts/Hour per Kill-O-Watt meter, battery removed, at 2.6Ghz during daytime, 2.7 or 2.8 during the night, but then eating 75-80 watts/hour]. The system switches autonomously up and down in steps of 99.8Mhz, based on 3 settings: power saving [1.9ghz], max performance, max cooling [which sets the fan to max, barely audible to me, but locks the cycles at default 2.2]. Notably the system switched up to 2.8 when the system had BOINC paused, and a DVD running], hardly ever seen it hit the max of 3.1 only done in short bursts, then throttled due the temps shooting up.

Not sure what was meant with having a 7200rpm SATA... would not mess with the innards of any newly bought laptop, but for maxing RAM. Given the 99.6% efficiency reached with 4 concurrent CEP2 of eight threads with a 5400 rpm drive, more rpm will not gain anything meaningful... not even an expensive wear exposed SSD.

And if you've got money to spare and time to wait for the ''newest/shiniest"... go for the Ivy bridge [same price as SB is said] probably hitting the consumer market in 3-4 months [meaning down here by X-mas]. Saw first tests and they do 8-9% more, cycle for cycle, but then is this a purchase to replace something old [my case 5+ years old 85 watts/hour duo and meant to be used for the next 5+ years] or just for crunching 99% of the time.

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edit: Oh, and it makes 1 iota of difference if running BOINC with 2 threads, 4 or 8, system control is based on hottest core, not the mean, so that was kind of a [had to be expected] bummer.

My 2720 run at 2.6 GHz with a fan pointing at the naked backside (backplate removed). It uses 65 W and does 3400-3600 PPD. With backplate in place and no fan it stays at 2.2 GHz most of the time. 100% dedicated cruncher when it crunch.