yeah i would go with a 350 minimum, but watch it, gateways have a tendancey to be proprietery on their power supplies, so double check on if an atx power supply will work inside that case. I've seen it many times before.......if it is, then you'll have to buy one directy from gateway or buy a different case and move everything over...... yuck! 8O
I am actually looking into this same issue. I'm looking to upgrade my graphics card to either a Geforce 7600GT or a Radeon X850XT to use mainly for Oblivion. The thing is...the 7600GT says that it requires at least a 350 Watt power supply, when I have only a 300 Watt...whereas the the X850XT says it only needs a 300 watt powersupply. The final total on Newegg for the X850XT is only $172 without a new power supply whereas the 7600GT with the powersupply is $233. My question is can I get by with a 300 watt power supply with the X850XT? It DOES say that is all it requires on the NewEgg requirements. But is bare minimum power supply a problem?
I'd say that you overshoot on any minimum requirement. So, if they say 350W, go for 400W. And seeing that the PSU is your life line in the case, don't neglect it.
As already mentioned, a good quality PSU you can try your luck on the minimum, but lesser PSU's might and most probably will give you headaches.
Lastly, always keep in mind that most PSU's advertise their max wattage output. So in the case of lets say, a 300W PSU, it nominal output is in fact 280W, but it can run 300W. So that, to me, is pushing it.