On my Wabook Sky, I did exactly that. I have two batteries hooked in parallel. It works well, and it uses the resistance feedback method of charging cutoff versus the temperature sensing method...so that lets them charge together evenly. With the two side by side, I get about 5 hours continuous use with wifi.
I also like the larger screen form factor, 1024x600 over the 800x480 of the smaller units. Makes surfing much nicer without all the wraparound problems. The drawback is that so far I haven't found any other compatible web browser that works.
The 8505 however is the way to go. I have customized my flash to run my legal copies of Textmaker Office 2006 and all of my other favorites...which means, hard reset and I don't have to reinstall anything. Everytime I add a software app to my netbook, I also add it to my rom flash to keep it updated.
I've pretty much customized everything from the logo boot screen to the sounds, replacing the default ce sounds to my xp desktop sounds to even customizing my log-off screen. I've even gotten used to that stupid damn chicklet keyboard....