Personally I find the ps3 pretty pants for surfing the internet- some sites don't display properly and anyway you can't,as far as I'm aware, downloadÿand playÿvery many formats, and you can't load software onto the ps3, unless you were to install another os onto it, but that all gets a bit involved.
Most pc's or macs will be able to do what you need nowadays, it's just a question of what you need from it connection-wise and quality-wise. E.g. video card capable of handling HD, or outputting over HDMI. Again I think there is a plethora of machines which would fit your needs- it's just a matter of trawling through websites or magazines. Of course, Vista premium and pro, or whatever they're called, come with windows media centre built-in, though how reliable this is at the moment I don't know. I have XP Media edition on mine, but it's not THAT slick or versatile.
Yes, it is fairly straightforward building your own home theatre pc- I have done it myself, but it just takes a LOT of research and possibly studying forums etc.ÿ
I think you have to do a bit more thinking about what you want, then research the options available. (I know you're trying to do that here-sorry if I'm not being much help!)