These days exploring hip hop is a lifetime journey, dom got a good jump off tho'... I know it's way late in the game but I had to put in my 2 cents on some overlooked isht and some of the stuff you were talking about.
Dom why don't you peep the movies "Wildstyle", "Krush Groove" and "Beat Street"... 3 80's masterpieces that'll give you a good idea of where it started at and an intro to the culture... then check some old school artists Africa Bambaata & Soul Sonic Force (oh, so thats where all those beasties samples came from), Grandmaster Flash, Whodini, etc.
There's a huge wealth of great tracks from the early days of hip-hop... Personally I love the electro hip-hop sound you can here on a lot of old tracks, mantronik, some old ice-t, prodigy, egyptian lover, jonsun crew and many one-hit-wonder type tracks like man parrish's "boogie down bronx". I highly recommend this site
http://www.unkut.com/?category_name=bootleg for some great hip-hop from all eras.
From there it all goes into Eric B & Rakim, RUN DMC etc, then the 88 golden era sounds, then gangsta explodes...NWA etc... miami bass rap 2 live crew... early 90's wu revolution, g-funk etc, then underground hip hop, club hip-hop, blah blah... I'm no expert but I'm a big fan of many styles of hip-hop.
A good way to hear the best of the past is old dj mixtapes.
I recomend mix master mike's "neckthrust one" for some late 80's / early 90's stuff...
D-Styles' "inspiration information" for some great electro-ish / original gangsta / miami flavours",
Rawkus presents "soundbombing volume 1" (evil dee is on the mix...lol... don't bother with vol 2 or 3) for some company flow era quality underground hip hop.
to everyone else re:def jux ... they had potential at the start but I think they lost the plot collectively... and EL-P is a talented motherfucker but he got too whiney... I still think he's got some good rhymes sometimes but i don't wanna hear about his absent father complexes and family life which is what I hated the most about fandam. Some nice tunes still sneak out of that label now and then... c-rayz walz has his moments, the perceptionists lp has a few gems, and love them or hate them... CAN OX has a new lp of a live reunion show with some nu material and unreleased stuff coming out soon... not getting my hopes up but i'm definately gonna check that one cos I think COLD VEIN is a straight up classic although it didn't take long for vasts head to swell up like a zeppelin and start leakin hot air allover the place. I love both their rhymes on that album but Vordul is amazing and was almost completely overlooked in the press.
ok i stop rant now.