Last time I looked at netbooks was earlier this year when the OH drowned her old one.
Ended up buying an Aspire One as it was the best around at the time. The CPU to have was the N550 – it was a slightly slower clock speed than some of the the N4xx ones (1.5GHz), but was dual core and considerably faster in practical terms. There’s an N570 around now too I believe; same CPU but higher clock speed. A quick google would suggest that the Aspire One has this chip in the “Happy 2” (ugh) series.
So, yeah, anyway. Budget an extra 20 quid for a 2Gb RAM upgrade and look for a dual-core CPU, would be my advice.