The best way is to get your lawyer to arrange for you to own the house and your girlfriend owns the land. At the same time they need to set up a 'usufruct' for you to legally be allowed to stay on the land for 30 years, this is entered in to the tabian baan (blue book). People will no doubt say this would not prevent you from being evicted if you ever split up, but it's the best way other than not buying a house. The other alternative is to buy a leasehold house on a housing complex, so you only ever own the house and someone else owns the land.
There are two ways of achieving this; easy/inexpensive and difficult/expensive.
Brandon and Lynette explained the easy route, Bill gave you the difficult option.
It used to be possible to arrive without a visa and either do it yourself or pay an agent, but that has changed now and it seems even agents are having to use workarounds like getting an education visa first, before getting the retirement visa, all of this adds cost. Up to you.
I have also used a different branch in the past for the letter and statement, so it's definitely possible. As always this is Thailand, so it's also possible a bank might refuse, so dress appropriately, be polite and hopefully you will have no problems.
After the first year, if you can show 12 monthly deposits of 65,000 you can change to that method, but you still need to satisfy the requirements for the first year, so I think you need to wait 3 months to withdraw the 800,000. Not done it yet, I plan to do it in a few months, so double check with your immigration when you extend.
you can withdraw up to 400,000 after 3 months (if I remember correctly) and then top it up again 2 months before your next extension. So if your balance dropped below 800k 1 month after you extend (even a small amount), you would not qualify for an extension the following year.
over a period of time the interest will, but if you deposit exactly 800k, after a monthly charge (I forget how much it is, but even 20 baht) would drop him below the 800k required and then he's starting the visa process again.
I have 2 accounts as Paul suggested and to answer your question yes you do have some bank charges, so always keep an additional 1 or 2 thousand in the account, so that it never drops below 800k.