working with incredibly large numbers in .NET

I'm trying to work through the problems on projecteuler.net but I keep running into a couple of problems. The first is a question of storing large quanities of elements in a `List<t>`. I keep getting OutOfMemoryException's when storing large quantities in the list. Now I admit I might not be doing these things in the best way but, is there some way of defining how much memory the app can consume? It usually crashes when I get abour 100,000,000 elements :S Secondly, some of the questions require the addition of massive numbers. I use ulong data type where I think the number is going to get super big, but I still manage to wrap past the largest supported int and get into negative numbers. Do you have any tips for working with incredibly large numbers?