I recently bought a HD4870X2 2GB card and with a Antec 650W PSU, all the power requirements are sound.
But when I booted up the machine the first couple of times (Windows Vista and Windows XP, dual boot) it would get to the OS selection screen and then just hang or in the case of Windows XP just reboot.
I originally had 3GB of Ram installed in my machine. 2 1GB sticks in dual channel and 2 512MB sticks in dual channel mode.
I just assumed I could keep them all in and windows would just ignore the extra 1GB of RAM.
But as soon as I took out the pair of 512MB sticks, it all started working again.