that cpu is old but it can still handle a gtx 550ti easily. but for 1024*768 i dont think you really need a gtx550ti. you would be more than fine with a radeon hd6670 gddr5. what psu do you have ?
your cpu will hold you back a lot and you won't be able to play many games very smoothly no matter what graphics card you buy. I suggest either upgrading everything soon or buy something less expensive.
So it means with less expensive card i can play games smoothly on that res. ? What if i buy 550 ti ? Even after bottleneck ,Will i be able to play games smoothly ?