You have to keep in mind that there are still people surfing the net with win3.1 and netscape 2.  Are there a lot?  Obviously not.  The point to keep in mind, though, is that for every new feature you have on your website, you're going to be losing a percentage of people who can view your site.

Is there a reason you have to use a .png over a .gif?  Unless it's absolutely necessary, stick with what is universally supported.

Of course, I'm a giant hypocrite who designs most of his sites in Flash.  I get to charge more for Flash websites, though, so I at least have a valid reason.  

The only real reason i would use a png would be for the alpha channel support and the better file compression than .gif files have.

However, Internet Explorer makes it amazingly hard to have it show correctly inside a browser, so unless your website caters to Mozilla, Opera users it would be better to stick to either a .gif or a .jpg.

(God. jpg 2k should be out and supported by now. Hurry up!)