Extension forms - When to file 2350

Sorry, to start a new topic on this issue.  However, I wanted to get this clear. 

I'm an Yank, married to a Brit.  Last tax year, PWC (Price Waterhouse Coopers) filed my tax return for me, as it was included in my relocation package.  Therefore, this year, I'm responsible for the filing.  I understand, I'm given a two month automatic extension from the IRS.  However, the language at the IRS website is a bit unclear (maybe just my understanding).  Do I need to file Form 2350, for the extension up to June 15?  Or am I entitled to it without filing anything?
 
And...If my Return is still not ready at the June 15 deadline, will I then need to file Form 2350?  Also, what is the difference with Form 4868

Thanks for your help!
 

Assuming you're in the UK, you file a regular extension, Form 4868.  You file it now to pay any tax, or any time up until 15 June 2006 if you don't owe any tax. 

Form 2350 is only used in the year you move, which was 2004. 

Thank you for the response.  Do you believe, I would be able to send 4868 w/o estimateing the total tax liability.  Or, should I use the same figures from 2004?  This year my income is solely based on UK employment and a US rental property.  As my federal return last year was 44 pages, it is freaking me out.