Most of the $ difference is in the Hard Drives.
Common are 4 GB, 16 GB solid state and 160 GB IDE which is the cheapest.
Almost all will come with 1 GB of RAM but hit Frys on 635 for a $35 2 GB stick.
Atom 270 1.6 GHZ processors are the most common bit 1.66's are popping up more frequently.
the 8.9 screens are too small for my taste, the 10 in screens are a bit better and the case is the same form factor/size as the 8.9.
Dell always has a ton in their outlet store but you need to check daily, sometimes multiple times, and grab the deal when you see it.
Best prices are on refurbs but you need to watch those. The Acers come out of a recert center in NJ and they are not thoroughly checked. Just got one a couple of weeks ago through woot and there were a lot of complaints of DOA's.
The netbooks are really for surfing, word processing etc... They will run movies but forget anout any serious gaming on them.
Some of the other price diffs will be things like Bluetooth. Personally if it comes without BT you can grab a button sized USB radio at Frys for $12.
The other nice feature to get if you use one to travel with is a GOBI card. It's basically a GSM radio with a SIM card slot that accepts cards from AT&T, T Mobile. You can get a data plan for the laptop without having to get a USB cell modem.
Doubt you need that for the kids though.
All have wireless.
Forget ebay.
Check daily,
http://www.dealnews.com and
http://www.bensbargains.net
They'll also alert you to the Dell outlet deals.