qgis - Cut many lines with many polygons: Too much for my CPU/RAM?

I have a line-shp (80mb) and a polygon-shp (5mb) and want to intersect them. In result, the lines (contours) should be cut by the polygons.

After using one of the tools "Vector -> Geoprocessing -> Dissolve/Clip/Intersect" to do so, the loading bar keeps at 0%. Same by marking just 10 of the 6000 contours to intersect them. The CPU usage is at 50% (one core 100%) all the time, even for more then 30minutes...

What is the problem? How to solve it? But don't tell me, I need a new computer! :)

My config: AMD II X2 240 ; 4GB RAM; xubuntu 12.04; Qgis 1.8

Answer

You could try Darren Cope's method for clipping large shapefiles, it uses ogr2ogr. See:

http://darrencope.com/2011/03/31/clipping-large-shapefiles-using-ogr/

Added later: Installing QGIS also installs ogr2ogr, so you already have this.

Nick.