with the 4650 it should game fairly well at 1280x1024 and below. anything above that you need a more serious graphics cards.
/new soap box
if you know what size monitor you have (resolution) that seriously helps figure out how much graphics card you actually need. however, more gpu means more future proof. however, if you spend $120 on a graphics card instead of $240. in 18 months, when graphics cards are much better, you can buy a new gpu for $120 that is better than the original $240 graphics card
at the end of the day its building a computer to do what you want it to do, and what you'll want it to do for the next year or 2. and about doing it for an amount of money you feel good about spending.
There is more to life than pew pew you know, like staying out of the fire so you can live to pew pew another day