What PSU is bottom-line for a new Radeon HD 4850?

I'm looking to purchase either an HD 4850 or 4870 online, mostly with my eyes set specifically on one 4850 I found for a little less than $100. I'm about to make the purchase, but what I'm worried about is my PC's power supply (PSU). It's already pretty good for gaming, but comes with a weak graphics card (as almost any PC that isn't an extra $600 just for that extra) and, my worry, the PSU being only 300 watts. These two graphics cards I'm looking at generally say they require about 500 watts. With only 300 watts, will my computer be able to function with it? Will it literally just not display anything if I don't have enough power? Or will it work, but when I wish to play games or something that requires a lot of power, it'll start getting wonky? And if this is all true, is it possible to perhaps combine PSUs, as I could possibly combine my current PSU with my old computer's PSU?