Not sure on the context of your meaning, "doesn't matter", but in any case it makes a huge difference to whether you can run a good GPU or not.
In any case, your looking at a 1GB GTX460, or HD6870 upwards in order to play at full HD and high settings.
Lower GPUs will play HD resolution, sacrificing your detail settings to do so.
You need at least 500-600W, from a reputable PSU manufacturer to get the best from these GPUs though.
SLI GTX460's, or CF HD6870's will provide for awesome graphics, but realistically you will need 700W or above to power them.
Do not skimp on the PSU, its the fastest way of destroying all the money spent on the GPUs, and in worse case won't even provide them enough power, or even worse still, will overload them and cause damage.
All of this assumes you have two free PCI-EX slots, and the ability to run SLI/CF, and a PC specification that won't actually bottleneck all the performance gains possible from them.