Can I replace pine nuts with other nuts in a pesto sauce recipe?
First, I'm assuming by "pesto" you mean "Pesto alla Genovese", given your question about pine nuts.  Basil, pine nuts, garlic, olive oil, and cheese is a delicious combination, but it's only one of many "pestos" (peste, actually), since pesto refers in general to any sauce which is made from crushed or pureed ingredients.  Mix and match to your heart's content.  

Walnuts are actually a traditional alternative to pine nuts for Pesto alla Genovese; pine nuts have always been scarce, even in early-20th-century Italy.  As a caution, though, you need to get good, fresh walnuts and make sure to get all of the skin off the nuts, or the pesto will taste bitter and rancid.

Almonds could also work, although I'd think they'd be rather bland.  The herb pastes which traditionally have crushed almonds -- such as Romanesco sauce -- include some hot pepper.  Again, freshness and getting the skin off is important.

I'd think sunflower seeds would be kind of odd and oily, but you don't know until you try.  If you do, post a comment to let us know!