Dual booting does not affect and is not affected by processor speed. Installing Windows Vista – dual boot or not – is a bad idea, though.
I would normally suggest the Ubuntu Netbook Edition, but 10.10 uses the Unity interface, which is just not ready for mainstream use, imo, and should not have been included in 10.10. 10.04 UNE would work very well.
An important proviso here is the age of the netbook, and specifically the data rate of its SSD – older,1st generation SSDs in the first batch of netbooks are extremely slow, and even the netbook editions of Ubuntu would suffer from poor performance. If your Eee is older, consider a distro running XFCE or LXDE – they’re very light desktop managers and will work far better than GNOME and derivative desktop environments on older hardware.
If your Eee is newer, I would recommend Ubuntu Netbook Edition 10.04.
If your Eee is older, I would recommend Xubuntu 10.10 or Fedora 13 with XFCE/LXDE.