Yes i´ve tried it.....
If i use arch=x86_64 my computer also restarts.....
The problem is that i really need this bootloader so that my graphicscard works without an enabler and work in 64 Bit...... Because the other way developed by netkas (aty_init) is just for 32 Bit......

Maybe the flag arch=x86_64 is renamed in RC5....
Would be really interessting because i want to boot into this mode.