Do you like BOINC? Care to try http://escatter11.fullerton.edu/nfs/ ?
HT on on this project. Set

lasieve5f - used for huge factorizations, uses up to 1 GB memory: yes

at
http://escatter11.fullerton.edu/nfs/...subset=project

You need at least 1.3GB per thread.

Forgot to add, set this

Swap space: use at most 90% of total
Memory: when computer is in use, use at most 90% of total
Memory: when computer is not in use, use at most 90% of total

at http://escatter11.fullerton.edu/nfs/...?subset=global.