yes, u can achieve non lagging gameplay by upgrading those parts. but, u might need to upgrade your motherboard also as your C2D E6600 use old 775 socket, compared to updated processor that use LGA 1150/1155. i5 4460 is good already for gaming.

What's your current PSU power and model?

your RAM is enough currently, but if got extra money, can opt for extra 4GB stick. If money is your issue, buy any cheap GTX 750ti (RM500-RM600+++), u might able to use your old PSU as those card use little power