I too have been just gifted an Inspiron 5100 Laptop.
This one has 512MB with a 30 GB HD. I just put win7 on it.
I will admit it's slooo. So I have been doing a bit of research. It seems that you can update the Bios from Phonenix(I hope to figure it out) and then up the RAM to 1GB.
I think it was really Gracious, and I thanked the person with Sincerity...Never look a Gifthorse in the mouth.
Here's a link to information for MEM.http://www.crucial.com/store/mpartspecs.aspx?mtbpoid=2e4edcdea5ca7304