I have this inside my computer:CPU - 2 quad q9950 2.83ghz ,ram - 4GB, motherboard - gigabyte ep43c-ds3 and power supply 504 WATT so i was going for NVIDIA gtx 650 ti and i was wondering if it is worth it only updating my video card ,will it significantly improve game performance? current video card nvidia geforce 9800 gt
Generally speaking, yes. The GTX 650ti scores ~4x better on the Passmark G3D Mark score. See this chart and search for the two cards.
This is no guarantee, but the 650ti is 6 generations newer and should be significantly better.