If you're just looking for a static web page (one that shows the same content forever and ever until you change it manually), then you'll be fine sticking with HTML/CSS. Once you want to have a dynamic web page (which almost every major site is nowadays, including this one), you'll need to learn a programming language of some sort.
I work for a startup, and we have basically three people working on the website. Generally speaking, one guy designs the pages (in photoshop), one guy builds the HTML/CSS and user interface, and one guy (me) handles the backend--the server, the databases, etc. The designer overlaps with the HTML/CSS guy to some extent, the HTML/CSS guys overlaps with my work somewhat, and I overlap with the HTML/CSS guy on some things. (as an aside, our web site is built in Java, but I also do a fair bit of PHP and a while back I *shudder* had to do some Visual Basic)
My advice? Give the HTML/CSS a bit more time, so your knowledge is a bit broader. Then go ahead and start learning some PHP. When you're building a php page, you can think of it kind of like a plain HTML page, with PHP determining some of the content.