Really depends on, as alway,s what the machine is going to be used for (and what your budget it) I assume you are looking for something general purpose at the lower end of the scale?
I had an older sempron once (in a desktop) and wasnt too impressed, and once (briefly - before it was stolen

) ownded a vaio with a celeron M and again didnt think too much of it - always seemed sluggish. - recently bought a machine witha core 2 duo (similar to
http://www.laptopsdirect.co.uk/Asus_...3C/version.asp, but with slightly different graphics card) and have been quite impressed (I dont do anything too intesive, only general usage/ occasional older games/ some programming) - in fact I have been quite impressed with the laptop in general, and dont even mind Vista too much (although I did have to put in another gig of ram (now with 2g) to keep it happy.
If budget is a concern, other ASUS f series are available, without the graphics card/ but thith a c2duo, and as with myself, it can be bought with 1gig and another 1gig added later (as long as you ensure its supplied with a free slot)
Anyway.. that be my 2p worth..