I\'m running up against the 2gb object limit in c# (this applies even in 64 bit for some annoying reason) with a large collection of structs (est. size of 4.2 gig in total).
There's an interesting post around this subject here:
http://blogs.msdn.com/b/joshwil/archive/2005/08/10/450202.aspx
Which talks about writing your own 'BigArray' object.