I've just done the same thing, bought a PS3 to upgrade me to Bluray. Even though i have a stand alone media player i have to say the PS3 is great, as not only does it play the stuff you chuck on it but also if you have a wireless internet connnection, you can stream from your PC. It also allows you to playback music off memory sticks and external hard drives as long as they are fat32 formatted

I've just upgrade my PS3 hard drive to a 320gb hard drive, very very simple to do and so worth it :)
As for PlayTv, have a look on the avforums, there's a little conversion going on about it in the PS3 forums and they know their stuff.
Not sure about streaming from a NAS but as i said before, you can stream from a PC or plug in an external hard drive.
The 40gb PS3 and the 80gb they're about to release can play back most PS1 games (a full list is on the offical PS£ website) but cannot play back PS2 games like the old 60gb. Depends how much you want to play PS2 games
