As mentioned above, avoid paypal's fees and less favorable exchange rates. (along with trying to use a credit card for this) Bank transfers are best. (I haven't used Japan Post, but have heard it's good, and also good for smaller amounts.)
For bank-to-bank transfers, I normally use Shinsei, but have used MUFJ a couple times in the distant past. Since it has been a while, I don't recall for MUFJ, but Shinsei seems to have a good link to the account that I send to (a major brokerage), i.e., there are no extra fees along the way.
Sometimes for these transfers, besides the upfront fee, some intermediary bank (or your receiving bank) will nick you for another $20-$25. You might want to check with your target US bank if they assess such a fee on incoming wire transfers. This could make a difference if you're cutting it close when paying bills. To be safe, at least until you know the system's charges for the routing and banks you are using, you should send maybe $40 extra--if you need $2000 in your account, send $2040.
With Shinsei, you have to do the first transfer in person, but there's a check box on the transfer form allowing you to register that receiving bank with them (given that you have an account with them here). Then, subsequent transfers to that same account can be done by phone. (I think you can only register one target account with them.)
In my experience, it usually takes two days, give or take. As with domestic transfers, if you put the order in later in the day it takes an extra day to process.
Finally, my Shinsei balance is generally high enough that they give me one free overseas transfer per month. But while it could have gone up, I think their fee is 4000 yen for an inter-bank transfer.