Thanks for the answeres.
about the memory the problem is I'm hitting this on only some of my servers but not all, one of my servers has only 3GB memory which 1G is used but on another 16GB/16GB is used.
The ngx_slowfs_cache module is usefull when the server has 2 type disks, SATA and SAS, but I have two servers which I want to do the proxying.
I'm looking at the included proxy module
something like this:

proxy_buffers 10240 128k; #needs 1.2GB memory
proxy_cache_path /dev/sas_disk/cache;
proxy_max_temp_file_size 2048m;

proxy_store is tempting:

proxy_store on;
proxy_temp_path /dev/sas_disk/temp;
root /dev/sas_disk/www;

is this what i'm looking for?