Need Advice for Computer Upgrade

[ramuji]

I'm considering upgrading the Dell computer I've used for work over the past 2 years into a gaming computer. The graphics card is clearly the weak point, but any additional advice would be appreciated.
My primary monitor is a 19" Dell monitor, but I would occasionally also play on my 46" Sharp Aquos.

By the way, for the most part I know little and am rather dense about building/upgrading computers, so explanations aimed at newbie-level would be appreciated.

Here are the details:

Dell OptiPlex 745 (http://search.dell.com/results.aspx?s=gen&c=us&l=en&cs=&k=optiplex+745&cat=all&x=0&y=0)
Intel Core 2 CPU  6600@2.4GHz
2GB RAM
Intel Q965/963 Express Chipset Family