Well, you need to forward ports used by the ps3 software. What I would do is to enable UPNP temporarily, See what ports ps3 opens. Then disable UPNP and open the needed ports manually. I guess that different games could/would use different ports so you might need to open new ports after buying new games.
Don't know if ps3 is "smart" enough to open ports only when needed(i.e. psp remote features). if so then you need to do more hunting to find out needed ports.
UPNP is risky as if some machine in your home network is compromised, virus/hacker/whatever can easily use upnp to open all ports. Opposite to if the router is behind secure password.