I would think you would be fine to run just that single card, without overclocking. Assuming your just running the basics, single hdd, optical etc. Rosewill are not too bad but to be on the safe side it might pay to upgrade to a cheapish good quality psu.
Also it is more than likely that the guy your talking about had a earthwatts 380w, and those my friend are one **** of a psu. I remember another member here when i had mine had his running a quad core 4870 rig with ease. They are made by seasonic and this is why they are so good.
4830 would your best choice, 4770 is a little better but it cost extra. you can pickup this for under 100 Newegg.com - SAPPHIRE 100265HDMI Radeon HD 4830 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card - Desktop Graphics / Video Cards
you can even get a 4850 for 99 after rebates which obviously would be a great buy.