I was going to suggest the new $69 Kindle, which I know handles Russian text just fine (we do it quite frequently, and have given away three Kindle devices to Russian language readers), but I presume that having to do format conversions is probably out of the question.
Given the requirement to read Russian text and the presumption that you'd like to be able to just drop the books on without any conversion I'd recommend that you look into readers by Pocket Book International, a Ukrainian company whose ereaders support fb2 and the other formats that you mentioned natively. They sell both 6" models as well as a nice big screen model.
http://www.pocketbook-int.com/us/products