so you really want to try fglrx. you should know that the driver is so bad that you have to patch libpciaccess in order to get it running with xserver 1.12 and debian 64 bit. i don't get why it works with arch.
the debian package has a fix for kernel 3.4 included, by default fglrx supports only 3.3. next thing is the hidden config string is discovered to use xvba with h264 l5.1, you need
if you use sudo (for ubuntu) or sux (for systems without sudo) this setting works on the fly. you need xbmc fernet menta branch to use.
if you want to see your system crashing you can try
i verified that crash yesterday with my hd 5670, my nvidia 405 had no problems. intel ivb crashed as well.
there will be games that work with fglrx, but you will find examples that run very badly with fglrx like rage via wine. games via wine run usually best with nvidia, but if you want amd then keep in mind that you knew it before.