Traveling from Ulan Bator to Novosibirsk, there are several interesting stops each worth several days -
i. Ulan Ude on the east side of Lake Baikal
ii. Irkutsk on the west side of Lake Baikal
iii. Krasnoyarsk a large city with a long history and some interesting sites
iv. Tomsk which is a university town with lots of old wooden buildings
Novosibrisk is worth a few hours and is probably the least interesting of these places if you ask me.
If you decide to take the overland route through Russia, I have lots of information and some contacts to suggest.
Also Almaty is worth a few days. Check the central asia branch for more information.
It is also possible to cross from north of Ulgit in Mongolia to Altai in Russia overland. There are no regularly scheduled buses on this route but it is beautiful. Once in Russia head north to Barnaul and catch the train to Almaty from there. The international border point is Tsagaan Nuur in Bayan Ulgii province. Traveling in Altai is VERY interesting.
To some degree your plans will depend greatly in the amount of time that you have to travel, your overall budget, your interest in travel in Russia, etc.
For flights from Ulgit to Oskemen and then on to Almaty, try asking on the Central Asia branch and searching for various spellings. Try contacting David Berghof of stantours.com for information. This travel agency is located in Almaty and the operator has been very helpful to TT users.
Ruth