Given that the block chain is 140gb now, how long does it take a node or a miner to verify that a transaction is valid i.e. Not giving negative balance for an address?

Also, given that the number of transactions is always increasing, I am assuming that the verification time increases as order(N) i.e. linearly with the number of cumulative transactions? Will this become a big problem in the future as it takes longer and longer to verify transactions?