Hi, I just built a new system with the FOLLOWING specs:

ASUS M3N-78 motherboard
AMD Athlon64 X2 5000+ processor
2 gb ddr2 ram
antec tp-II 550 watts power supply (not new; from a previous system i'm replacing)
seagate 320 gb sata drive
seagate 200 gb sata drive
lite-on dvd writer

I use an APC ups with an output capacity of 300 watts. I use the onboard video of the motherboard and it works just fine. Just yesterday, I tried to add an Inno3d 9600 GT Overclocked edition videocard but when I start to power up the computer, it will turn on and just turn off. When I try to turn on the power SWITCH it wont power up again. I would unplug the PC from the APC then plug it again and turn on the pc. Then the APC would start beeping and there's no power to the pc. I know that my power supply is good enough for this system. I am thinking that it might be the APC cause its only 300 watts and might not be enough for this system. COULD this be the case? For the moment, can I just use an AVR on my pc rather than a UPS? Right, now I'm not really concerned with power backups, just stable power.I'd go with a 750va UPS surprised that it ran ok before video upgrade on just 300va bottleneckI don't know what AVR has to do with your situation. Anyway, have you tried simply plugging your computer into an outlet, at least long enough to confirm it will power up when not connected to your UPS?