dvn, along with the issue about RAM as mentioned above, you should also consider your "end user".
You don't mention how your end product will be used but if you are sending it to be professionally printed you should check that the print shop can handle documents of the size you'll be producing. This forum topic may be a useful read: http://forums.scribus.net/index.php/topic,953.0.html
On the other hand, if you are expecting people to download your PDF from the internet you should keep in mind that a lot of people don't have high-speed "all you can eat" internet access.
For some people a 50MB (for example) download might only be a few seconds wait, but for others they could be looking at many minutes of waiting or worse.
If your end product has a very specific "market" where you can be pretty sure that the problems I've mentioned will not come up then that's fine. However, if you are wanting to distribute something by PDF download across wide geographical or technological spreads then you'll need to think about how your end users can access the material.
For instance, if you were making a book to give people information about how to recognise the dangerous/poisonous animals and plants of the world (something very useful to everyone), you could make it one big book but if it's too big for someone to download then they can't get the benefit of reading it. On the other hand, if you split the book into separate volumes, each for a different country (or whatever), then each document would be much smaller and easier to download - and thus more accessible and useful.
Also, if you are having memory problems when composing your document you should think about the computer resources available to the people who might want to read it. It's easy to forget that a lot of people only have access to very limited computers with minimal amounts of storage - both RAM and disk. (And what about people reading on tablets and smart phones? It's worth thinking about.)
None of the above may be issues you will personally encounter with your current project but I thought it was worth me saying something so that other forum readers are made aware of potential problems that could come up with their own work.
A good question to always keep in the back of your mind when you're creating anything is: "What's my target audience and how will they access my work?". If you keep asking yourself this question it will help to guide your progress in the right direction.
Anyway, that's enough waffle from me, for the moment anyway.
I hope you get your photo-book completed and it is well received.