Oh sorry I haven't answered, came back from Bulgaria this morning! I honestly don't think it will be worthwile getting a Interrail card if you're only doing Eastern Europe - I think you're better off buying single fares. For example, Budapest - Belgrade was about £15 when I was there a month ago. Also, keep in mind that buses are usually more popular than trains and departures much more frequent. Best advice I can give you really is to stay a couple of days more in the same country rather than rushing on to the next one - when I did it I felt a bit like all cities were the same as I didn't really have time to explore them. Your schedule looks a bit tight if you're only going for two weeks. Any more questions just ask.