Since corn sugar is nearly 100% fermentable, it will increase ABV but also dry out the beer and thin the body (relative to the amount of alcohol). DME is around 75% fermentable, so it will increase ABV but proportionally leave some body and residual sweetness.
It's usually not the best idea to simply change the gravity of a recipe without changing the hops, specialty malts, etc. to help balance it. NB designs the recipes based off certain IBUs, BU:GU ratios, grain percentages, and other factors. So by boosting the OG this way you're changing the malt presence in the beer and throwing it slightly off balance compared to the original recipe. Not necessarily a bad thing, but definitely different!
It sounds like you're starting to play around with your own recipes, so I'd recommend checking out the free trial of Beersmith! It's easy to mess around with different ingredients/volumes/etc. and see how the recipes change. You can also pick up "Designing Great Beers" which gives a great overview of recipe formulation for most beer styles.
Finally, the math for OG is pretty simple when you're talking about extract or sugar. If you have the Briess Golden DME (http://www.brewingwithbriess.com/Ass...enLightDME.pdf or http://www.northernbrewer.com/shop/briess-dme-gold.html), it's listed as 43-44 ppg. That means if you added 1.5 lbs, you'd add 44 * 1.5 = 66 total gravity points. For a 5 gallon batch, that should add 66 / 5 = 13.2 points to your OG.