Quick question about wallets On my current OS I have daemons of bitcoin and other altcoins running in bitcoin there were 5 different addresses lets say 


userA
userB
userC
userD
userE


userB has sent some coins to a address generated by him, and this transaction was covered in e.g block number 300 but my bitcoind was synced till block number 250 only. Now if I save wallet.dat and place it in the same folder after reinstalling the OS will this retainn the addresses generated by different users ? and after synchronizing to block 300 will those coins comes into the wallet ?
Your balance is kept in the cloud, specifically in the block chain.

Your wallet does not contain any balance info but just a private key that you can use to unlock the funds inside the block chain.

This means that it doesn't matter that you backup your wallet.dat file before your client is fully synched. Since it only contains your private keys, and not the balance itself.

So yes, after a full synch is completed, those coins will appear in your wallet. As long as you have your wallet.dat file, with the private keys associated to your address, you can access your funds.