Best answer as chosen by user budbd: More than likely

More than likely, it's your old P4 CPU that's not up to the task. Video decoding is primarily CPU intensive, and these days it's pretty much assumed everyone has at least a dual core CPU.

A new video card MIGHT help if you get one that's capable of doing hardware video decoding with Flash, but given that unit probably doesn't have anything more than PCI and AGP slots, you're going to be very limited in selection for video cards.

Personally, I wouldn't waste the money on a computer that old. Take whatever you were going to spend on a video card for this old computer, and get one of those set-top boxes everyone is making now days. Western Digital has something, there's the Roku, and probably at least a dozen more. You walk through a big box electronics store and you should see plenty of options. Several of them can stream content from YouTube and Hulu, along with some other sites, and you'll save a decent chunk of change on your utility bill that way too, plus they're a lot closer to "instant on" compared to an old clunker desktop from 2004.