I know the feeling
I used to have the 70-300 IS and a cracking lens it is. I would probably have kept it but I needed an extra 100mm for sport so sold it and bought the 100-400. So that is the first question to ask yourself....how much do you need 300mm?
Secondly, I used to have the Tamron 18-200 which, as Dabhand correctly says, is an excellent walkabout range. But without IS it doesn't really cut the mustard in low light so, bearing in mind I was covered from 100mm+ I looked at the lower end for a walkabout. I bought the 24-105 L IS and it is superb.
If you use the top end of the 70-300 then keep it - if not then I would suggest that good glass is worth the investment and would recommend the 24-105 and then, maybe later on, something around 10-20 and then you would be really covered
