There's no authentic limit in the Data Loader itself.
From the Data Loader guide:
Use the Data Loader when: You need to load 50,000 to 5,000,000 records. If you need to load more than 5,000,000 records, we recommend you work with a Salesforce.com partner.
Really check out the PDF, especially for the (little) info about "bulk API" if the speed of normal loading is insufficient for you and you have already tweaked the number of records sent in one batch (by default 100 or 200 I think).
And if the bulk API looks too complex, you can always use very simple parallelization of the task - one username can have up to 4 sessions open, so you could invoke Data Loader 4 times with 1/4 of original CSV.