Realistic PSU calculation?

So my original PSU for my pc died a few days ago and I've been looking to replace it. The main components in my pc are as follows:

AMD Phenom 9600

2x 2 Gb DDR2 RAM

ATI HD 4850

ASUS M3A mobo

I also have 2 DVD drives, a single 7200 rpm hdd,and my mouse is powered via USB. The calculator at this website ( http://extreme.outervision.com/PSUEngine ) tells me that a system with my specs only requires a PSU with 304 watts. However I was at a hardware store yesterday where a salesman told me I wanted 550 watts bare minimum, and recommended that I go for 600. Which of the two seems more plausible? I am by no means an expert on power issues.

It depends on the 12v rail. You want something like this http://www.newegg.com/Product/Product.aspx?Item=N82E16817139004

Would 550w be sufficient? I was reading the other thread about the Amd Kuma and I'm seeing the "rails" mentioned a lot. Apparently that is a limiting factor? Am I really looking for wattage here or the output on the rails? I would be figuring this out on my own if I had the slightest idea what I was doing :/ I'm guessing that website's calculator wasn't accurate.

On a side note, I found out my old PSU was a measly 230w, probably explaining why it died on me. However, it ran my hardware fine for over 6 months.

Something such as this: Here

Although 650W would be more than enough, If you cant afford that, try looking for a 500W corsair.

Sorry Didn't see the power supply the other guy suggested.

That PSU calculator seems to be pretty close to what thevariousbenchmarkssay. I power a system similar to yours (Phenom II 940 and power-inefficient HD 4830) with a Corsair 520HX without any problems, and I used to run it with a 380W Antec Earthwatts, also without problem (except for the fan noise). If you planned to never swap out your processor or graphics card with higher-wattage products, you could easily make do with something like a high-quality 400W PSU (this one, for example). However, if you think you'll get components that suck up a lot more electricity than what you currently use, then a high-quality ~550-600W power supply would serve you well.