I've been dealing with bulk data recently, CSV files of around 500mb or more. These can seriously stress out your pc. In the process of trying to write data import scripts I found an application that has proven its worth a few times over. This application allows you to split text files at points of your choosing.

File Splitter Pro, from wonderwebware. It doesn't look amazing, and it has a relatively simple interface, but after going through three or four alternatives that for the most part couldn't even open a 500mb csv file this has come through every time. What I really like though is the function that allows you to select N number of lines and move them to another file.

There is a trial version available too:

http://wonderwebware.com/file-splitter-pro/

(P.S I don't get commission or anything.)