Griffdowg
That brings us to the road through Russia. I haven't personally taken that route as the Georgia border crossing into Russia was closed when I went through, so didn't comment on it before, the issue with going that way is the Russian visa.
I don't know what your route or itinerary is, but if you're going to Mongolia (I notice you mentioned Siberia) via the 'Stans AND you take the Russian/Kazakh route to get around the Caspian Sea then you will need to enter Russia three times, assuming you will also be driving out of Mongolia. Possibly also a double entry Kazakh visa, or just get a second visa along the way which I have done before.

The most common visa travellers get for Russia (if going to Mongolia) tends to be the three month double entry business visa, which means you can enter Russia twice.
If you're going to take this route through Russia you need a different visa, I know Colebatch tends to recommend the multi entry extended time visa so that travellers will have the extra flexibility it provides (though at a price).
Take a look at the Russian visa options to see what works for you. The time limit on the Russian visas can also mean you have to travel more quickly than you may want to.

Some people have two passports, purely because of the Russian visa requirements, leaving one passport in the UK to get the Russian visa at the last minute. Then having it couriered out to them - generally £30 to get your passport sent anywhere in the world within 48 hours.

Does that make sense? Take a look at the routes etc.

Good luck

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Tiffany
On the road from...I'm not sure any more
http://www.tiffanystravels.co.uk
http://www.youtube.com/watch?v=XAUhV1r-kUo