If I have budget for one of two graphics cards as an upgrade, where one has more RAM but the other a faster clock speed on the GPU, which would have the biggest impact on gaming performance? The one with more RAM, or the "faster" one? All other factors should be considered equal (drivers, chipset, manufacturer, price, etc).
You need RAM to hold the data - polygons, textures etc. More ram will allow you to show more detailed textures and run at higher resolutions.
You need a good GPU to do all the calculations needed for a modern game - lighting, physics etc. The better the GPU the more of these calculations you can do so you'll get better effects and it will have an impact on the resolution too.
Check the specs of those cards and then you need to decide what's important - resolution, effects etc. and go for the card that's better at those things.
Having a big amount of RAM could help when playing at big resolutions, let me say
For resolutions like
If your monitor is
ps. Clock frequencies can be overclocked, RAM amount cannot be increased ;)