Not sure if you are in North America or not--if not I would think the 14" Timeline with Intel dual core would be the way to go. Reports I've seen indicate the single-core has trouble with HD Youtube content (aka Flash is a piece of crap).
Unfortunately they don't seem to really offer the dual-core models of the 14 & 15" in N.A.
Yep, North America, USA specifically. Haven't seen any dual-core versions of the Acer Timeline, though. All the ones I saw either had the SU2700 or SU3500 and as far as I know, those are single-core. I was eyeing the Asus UL30A with SU7300 but when I got home last night, Amazon was already out of stock. I'm hoping they're just clearing stock to make way for versions with Windows 7 pre-installed.