My stock gpu is 9800gt which is half broken because I couldn't use its full capacity. When it's enabled the desktop goes black screen of death, it goes normal when I disabled it but I couldn't play games. My suspect is overheating because I'm a hardcore gamer. I play games like call of duty 4, fallout 3, arma 2, etc. everyday and my pc is 1 year old. I use it everyday.

Now I'm planning to purchase the XFX - ATI Radeon HD 4890 1GB DDR5 PCI Express 2.0 Graphics Card (500 watts)

My pc specs is

Psu: 460 watts
processor: Phenom-9850 (A) 2.5 GHz (125W)
HD: 750 gb
ram: 6gb
os: vista
gpu: GeForce 9800 GT 512 MB GDDR3 memory
link: http://h10025.www1.hp.com/ewfrf/wc/d...roduct=3747345

Now I want to know everything about xfx Hd radeon 4890. Is it good for gaming? How's the cooling? when did it come out on the market? will it fit in my casing?
It's 500 watts, can I buy 500 watts or should I buy 600 watts? any psu brand you can recommend? include the price please thanks.

link of the gpu I plan to buy: http://www.bestbuy.com/site/XFX+-+AT...&skuId=9566025

It isn't the wattage that is important as much as it is the quality of the power supply. Get a good one as opposed to a cheap or no-name brand, unless you already have a good one such as Seasonic, OCZ, Corsair, PC Power and Cooling, or one of the other top models.. The video card is fine.