There isn't a wide range of Bitcoin vendors on Silk Road, but as others have said you can use LocalBitcoins and use the "Cash Deposit" option, all you have to do is initiate the transaction and the Bitcoin get held in escrow then the vendor sends you a message with their bank details and you just go to your local bank branch and deposit the cash directly into their account. It's pretty much instant, remember you have to go to a branch where the vendor banks with.