average lifespan of a mac? well my 2005 model still runs if youre looking at longevity. usually because of the soddered parts and very limited customizability macs will last far more than a pc just becaus eof how well the parts fit together. other than ram and an external drive being the things you can change the most, id say itd be better to save up and purchase a laptop with a nvdia graphic card thats at least 700 or above with i5, 1 TB and 8 gbs or ram.
most graphic intesive games or games that have alot of "junk" running in the back will require 8 or more gbs of ram but id say for now youll be okay. ram is an easy price hike when it comes to buying parts.
my laptop ran me 1700 and i have 16gb, but thats because my work requires me to use Maya and other CAD type programs which really eat at your processor speeds and can heat up your computer.
idk what your budget is antwan but if you want something that is fairly good and you have the money for it id check out http://www.pro-star.com/ , they have a good range of prices. though dont expect anything 700 and down yanno. the parts that they use for the base models of every comp are really nice, AND if you feel like uping the price to get some extras you can, though i think with what you might be wanting you dont need it all right now, or at least in a laptop. better save that for tower. but yeah check them out if you feel like it and just browse around. most of the parts n stuff you could find on newegg but i found that looking all over, certain sites just bait you into paying more than what you want.
and also you dont have to throw in an OS if you alreayd have a windows 7 or 8 activation cd. just a way to save more $$