My Dell optiplex 745 desktop has:
Core 2 duo 6400 @ 2.13ghz
3 GB ram(2+1)Ddr2
2gb GeForce GT 610 ddr3 graphics card and 280 watt psu
So please suggest me if I can max up my pc with 16gb (4X4) Ddr2 ram to improve the performance.
8GB Ram is enough, 3 (2+1) won't work well. Only 2GB are in dual channel mode. It only takes low density ram (chips on both sides of the modules). X6800 2.93GHz dual core will help, QX6800 2.93 Quad core also. Q6700 2.66GHz is cheaper quad. It can be overclocked with a Core2Extreme CPU like QX6700 or the other x chips I listed.
Better GPUs are GT730, GTX750Ti, GTX1030, GTX1050 2GB, GTX 1050Ti 4GB. Alll of these come in low profile, and will run off of your existing PSU.
QX6700 2.66GHz unlocked CPU are going for $20. It will run 3.33GHz easily with Throttlestop software and a D9729 cooler.