Best option is to build up from a minimal one. Either Debian, GRML, Arch, Crux is a good bet. You'll need to do your homework to know what kind of hardware you have (especially wireless chipset) and it'll take some time. If you do it properly and install a lightweight WM, it'll fly. Learn to compile your own kernel and do some undervolting with phc to maximize battery life and lower temperatures on your netbook.

Gentoo and sorcerer work great too, but require even more time.

If you want something out of the box with all the Gnome bling-bling widgets and effects, try Fuduntu : http://www.fuduntu.org/

Other options,

Zenix
http://zenix-os.net/index.html

CrunchBang
http://crunchbanglinux.org/

Aptosid (Xfce version)
http://aptosid.com/

If you want something ultra-light and small, but without as many apps as the big distros, you can also look at Slitaz: http://www.slitaz.org/en/