To answer your question, overclocking is not ever necessary so long as your cpu can do the job.
In the case of the G3258, which I love for a budget build, Intel gave it an unlocked multiplier.
That lets you increase the multiplier from the default 32 to somewhere in the range of 40 to 45 depending on the quality of your particular chip.
That gives you a 25% boost. Or, a 40% boost with a better cpu cooler.
Comments on your build:
1. 4gb is ok since you are on a budget. But, buy a 2 x 2gb kit so you can run in a faster dual channel mode.
2. You did well to plan on replacing the logisys psu which is considered crap.
But, why not buy a decent case in the first place.
One option might be an antec case with a decent included psu:
http://www.newegg.com/Product/Product.aspx?Item=N82E16811129202
Or, this looks to me to be a nice inexpensive case:
http://www.newegg.com/Product/Product.aspx?Item=N82E16811353050