Hi,
If you don't have already a power supply, you should definitly get a 300watt, just for the heck of it, you will be able to get by with a 250, but why risk it since the price difference isn't much. If you look at my sig, I have a bunch of high power crap, and not included are a bunch of fans, a dvd decoder card, etc. way more stuff, and I was able to run it fine on a 300watt, now I upgraded for the heck of it cause I gave my old antec 300 to my bro, an enermax 330. But the antec 300 was more than enough.
Alex