thanks everyone for replying.
it turned out that my PSU is 550 watt, and as for the brand, see this picture I took with my cell phone camera:http://img521.imageshack.us/img521/1231/photo10xj.jpg
and for the GPU-Z log, I couldn't figure what should I look for, so here it is:
please note that I played BF3 for 45 minutes before I upload this log, and the PC didn't crash this time.http://www.mediafire.com/view/?oad0jgqgmbdo642
and the result I got from newegg calculator is 447 W