I played this game a while back, and really liked it. When I went to order it, I realised that unfortunately, I forgot the name of it. Normally, when I do that I just wander around Electronics Boutique until I find it, but they don't seem to carry this one.
Here's the details -out on the PC -genre: RTS -you pilot robots on a multi-layered map (sky, ground level, and below ground), and can pick up parts from other robots you destroy, adding them onto your own robot -has an online play feature