First, don't buy from best buy. Shop online at www.newegg.com for better prices.
Second, that video card isn't a gaming card
Third, a more powerful video card will likely need a more powerful power supply
Final thoughts: honestly, your system is too old to dump money into. DDR memory is two generations ago and it has a Pentium 4 Prescott CPU which is horribly outdated and blown away by the even cheapest processors out there now.
I suggest you save up your money for a new computer. Sorry, it just would be a waste to upgrade IMO.
Get a decent power supply with healthy 12v amps like this Corsair 400W $50
And spend $100 on the best video card you could buy like a 9800GT.
I still do not recommend the upgrade. But it can be done. Your CPU would probably bottleneck the graphics card. But you could carry on the new video card and power supply to a new build if it doesn't work out for you.