Speed concerns with OOP and PHP4

I'm working on a large VB/Sql Server to Php/Postgresql conversion (approx. 60k lines of code). I'd like to design this thing as OO as possible, because I think ultimately it leads to better, more extendable and readable software. However, since php does the whole copy thing with objects, I'm a little concerned. Is this "deficiency" on the part of PHP 4 something I should be concerned about? If I have a 60k loc fully object-oriented php app with a large amount (approx. 500 or so) queries to a db, am I making a mistake by going OO? Will it really slow down the app *that* much, or consume memory like a pack of hungry piranhas? Thanks for the input, guys. John