Eh, not much more. It depends on who you sell it to, i suppose. A pentium 4 in a notebook is a big turn off.
But, if you can find someone to buy it for $600, do it!
Thats true, battery is not more than an hour, but for regular computing needs (internet browsing, word processing...and such) this could serve as a budget desktop replacemet with a memory and HDD upgrade, right?