Thanks, although none of those tips seemed to be useful... but it did acidentaly lead me there. After running the main virtualbox GUI (I usually launch the VM directly from the command line) I noticed that it shows a VBoxGuestAdditions.iso listed as the image for the virtualized cd/dvd-rom drive. All I needed to do was "mount" it in xp. This wasn't very obvious (most buttons say something after you click them), but I probably should have seen it. Anyway, it seems to have installed from the cd image.
Now I have some issue with some mouse and keyboard events getting permanently stolen from X, but I haven't looked into it yet. Good odd I'll figure that out in a couple of minutes.
edit/side-notes: that glitch seemed to be a one time problem and went away after restarting X... maybe realted to the windows key... pressing it in XP still doesn't do what't it's supposed to even after fixing the virtual console bug.. oh well.
As far as 1680x1050.. I finally got it... must resize the window manually.. some of the usual methods with the mouse don't work, but at least maximize does, or go better yet go into full screen mode. Then hit resize automatically in the machine tab.. everyone but me probably knew that already.
I also just find "seamless mode" which is pretty impressive and kind of makes it like full screen but better... if I can figure out how to make all the task bars all coexist peacefully.