Even C5 is going to have issues with only 512MB.  Recommended minimum for either to do any serious work is 2GB.  For either you probably are going to need a big swap file, and it will be slow.  When I was still running 5 Pro, I had an Athlon 64x2, with 2GB and it still got slow if I brought in any poser content.  Now I run a Phenom 4 core CPU and 4GB (3.5 usable) RAM and it flies, and all memory problems have pretty much disappeared.  I am not familiar with a Lexington2 motherboard.  Who makes that one?