It's not really the PSI, it's the GPM. For decks and walks I would recommend a 4GPM machine. Something with about 11HP. One of my small machines is an 11HP, 4GPM at 3000 PSI. It's great for what you are looking for. Northern tool has great deals on these machines. The stuff you can buy at home depot and lowes are junk (trust me, I do this for a living!) With a 4GPM machine, you can add down stream chemicals to do a much better job of cleaning.....or you can just hire my company to come out and wash your deck, walks, house, gutters and roof if need be!...only kidding....
here is a PW I would highly recommend for your applications.
Washing a deck is a bit more complicated than it sounds, if you would like more info on how to do it, just PM me.
For concrete or paver walks, you will want to pre-wet it, then apply an algaecide / fungicide, then blast it! You can get Krud Kutter at Lowes, it works ok. I make a 20g mix with 40% sodium hypo chloride 60%water. Then I add 32 oz of soap (krud Kutter will work for you), and a scoop of Arm&Hammer laundry detergent (mixes with bleach (hypo cloride) without gassing off, for a surfactant. Apply with your soap nozzle (the black one) and let sit for about 10 minutes, then wash off with a 15 degree nozzle (the yellow one).
Just spraying it with water takes the green away, but doesn't kill the colony. Like I said, if you want any more info, just ask me. I do this stuff every day.