Well, Last time I upgraded Slack, was back when I had 10.2 and 11 came out. No trouble there with the upgrade, the instructions were pretty easy to follow.

On this other upgrade, the instructions are still pretty clear to me, since I've followed the steps correctly.

Well, here's my pickle anyway. I got a Dell Inspiron 700M, it has a 855 graphics chipset. I'd use to run a program called 855 resolution patch so I could bring it up to 1280 x 800, which is my normal resolution. Now, Slackware 12 has Xorg 7.2, so the patch thingy is not even necessary, I just load up the i915 module and I get my resolution fine. BUT. When I close KDE or XFCE and boot it up again, my screen goes small on me. The wallpaper still looks 1280 x 800, but my working screen looks like 1024 x 768.

For me to fix this (temporarily) I have to become root, run xorgsetup, then run xwmconfig as root. Then, become a normal user, run xwmconfig again, change from KDE to XFCE, and then back to KDE (so I'm running xwmconfig twice). I don't understand what's wrong... I remember I changed the *.new files for X during the upgrade, and all my packages are correctly installed. Has anyone encountered a similar problem?

Oh yeah, I think I know what the exact error is. KDE thinks I got 2 displays working (display 1 at 1024 x 768 and display 2 at 1280 x 800). It's so annoying, I don't know why it's happening in the first place. Please help.

I am assuming you mean stop/restart the X server?

have you tried closing KDE/XFCE and then:
editalso what does /var/log/Xorg.0.log && Xorg.0.log.old show? (you might even diff -u them)