For a full featured DE your best bet is probably xfce. lxde is nice and light but the way I see it, if you're going to do openbox, do it yourself!
We've got a laptop that's just used for browsing/tv catchup and the occasional movie that was running on a single core celeron 550 (2GHz) and 1GB of ram with XFCE for the better part of a year that never struggled, and that was with practically no hardware graphical acceleration (SiS 672, terrible terrible old chip).
Then again, there's no harm in trying them both and seeing which one you prefer, pretty sure you could try a live usb with xubuntu or lubuntu to see which one you prefer before laying it down on bare metal.