Like XavierMace says, read up as much as you can. You won't regret learning about this stuff.
That being said, this is what I'd do -
Get an Ameetec Lower for $200.00 (Probably $216ish after tax)
http://www.ameetecarms.com/store/index. ... ucts_id=11Buy a Stag Upper from AIM Surplus (Its a new upper) for $475 ($487 shipped)
http://www.aimsurplus.com/acatalog/Stag ... Rifle.htmlPut the lower on the upper and you'll end up with an AR15 for around $703.00 that should keep up with you for quite some time.
Stag is made by CMT, who make good stuff. The upper is a 16†M4 profile barrel (ER Shaw, 4140, Chrome Lined) with a 1/9 twist, & a 5.56 chamber. It will have a few nice things that some other comparably priced uppers do not, like the ARMS 40A2 flip up rear sight, side swivel sling mount and an F Marked Front Sight Base. I suggest the Stag upper over the Ameetec offering because for around $55 more, you get a higher quality barrel, and a correct front sight. (I don't think Ameetec uppers come with a F Marked FSB)
You won't end up with any magazines this way, but they can be purchased as well.
http://www.globaltactical.com/axami/sho ... 32&prd=333Edited to add -
http://www.ar15.com/forums/topic.html?b=7&f=22&t=335496Theres the same upper for $489 shipped.
Mind you this is just my opinion.