It depends. Sites like http://www.abtests.com/ or https://whichtestwon.com/past-tests (latter is quite commercial) will give you an impression how even very similar tests will have different outcomes.
Studies or best practices will give you some guidance and a good start. In order to really optimize your site (and conversion rates, that's what you're looking for here) you will need to (A/B) test. Google Analytics and Google Website Optimizer can help a lot.
Thoughts:
- If it's a non-profit organization and you want people to donate - think about what's really required vs. what's optional (e.g. address for donation receipt might be optional, thus fields do not need to be shown in 1st step)
- Single page forms are often long and appear more complex - typically you see a high bounce rate here (users abandoning the site/form in the first step because it looks like a lot of work to fill the form)
- With more steps you will loose users in between (errors, loading times) but in total it might be less than in one long form as the small chunks are easier to 'digest'
- One way to tackle this: (technically) load everything at once to avoid the sever round trip in between but visually split it into more steps using javascript (e.g. accordion form)
- If you've got some time and budget then have specialized forms for certain use cases (e.g. membership sign up different than one-time donation) to address each case individually
- Again, testing is key - maybe start with wireframes and guerilla user testing to develop some hypothesis and later A/B test them
Always worth a look is Luke Wroblewski's book on form design. His website also has lots of insights.
Again, it really depends on your site, context, target users, design etc. and testing is the only way to get evidence. The web is full of examples and serious studies usually put a disclaimer into their findings that it worked for their setup but might not necessarily be applicable to other sites.
