I was moved to try Emacs with my own test file and (somewhat to my surprise - it's normally pretty crap with long lines) it had no problem loading a 3GByte file

I was moved to try Emacs with my own test file and (somewhat to my surprise - it's normally pretty crap with long lines) it had no problem loading a 3GByte file consisting of 3,000,000 1K lines. For short lines it's generally fine. I've never had a problem working with multi-GByte log files with 100-chars per line or so. (Even if it's OK for text, you are probably best off avoiding very large source files - some of the major modes seem to be written assuming <100K files and/or short lines. Python mode in particular seems to bog down very easily. C mode is fine for editing, but large-scale reindents can get pretty slow. And so on.)