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Well, just like a chain, it is only as strong as it's weakest link. So if you go from USB 2 to USB 3 with an adapter, it will only be as fast as it's slowest link, which will be USB 2.
 
USB 3 is about 10 times faster than USB 2. Google helped me find that answer.

"10 times faster" on paper
USB 3.0 is capable of transfer speeds of up to 5Gbps (gigabits per second); that's a little over 10 times faster than USB 2.0's 480Mbps (1,000Mbps equals 1Gbps). In practice, it won't always be this much faster — mileage may vary depending on hardware configuration — but it will always be much faster than USB 2.0

Also check out this page:

SuperSpeed USB 3.0 FAQ

Just remember, Mac's do not have USB 3. So if you plug a USB 3 device into a Mac, it will only run at USB 2 speeds.
 
Hopefully this little diagram I drew up will help you understand it better. USB2 to USB3 adapter would be pretty much useless simply because USB3 interface is already backward compatible with USB2, it'll just be bottlenecked by slower USB2 speeds.
 

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Have seen Sonnet 34 express card advertised to work in Mac 17" express card slot for going to usb 3.0. Has any actually tried that one yet and did it actually work?
 
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