Hello, I'm afraid I've just done something really bad ^_^"

I have got a Radeon 9200, and I wanted to use proprietary driver, since open are quite slow, just rolling firefox takes my cpu to 100%... Anyway, I downgraded the following packages:
xserver-xorg-core, xnest and xvfb

and now, when I boot, only text mode is available... I re-updated them using apt-get update, but nothing was solved... What can I do??? T_T

first we need to know how you reverted xorg. Did you use repos from 7.10? or did you install from another source. As long as you installed .deb files simply bring up aptitude, go to the obsolete or locally created group and remove all the older xorg files. If they are not there remove the xorg stuff from the x11 group. You might also dump a lot of your graphical stuff too, like kde or gnome, which ever you have. You might want to consider just doing a clean install over the present one, and just save your /home dir (Provided it's on a separate partition). Mixing older repos is never a good idea. It's like mixing testing with stable or unstable with testing. A lot can get borked. Live and learn.