My favorite part to spend more money on is the trigger, because having a good one can well...make it feel like you're not shooting a crappy ar15 trigger. It is the upgrade you will 'feel' the most.
I like the LaRue MBT series, they're great for the money.
Geissele also very good, usually more expensivethan LaRue.
If I had 1k to spend I'd probably buy a spikes or psa lower, LaRue mbt, and a Bravo Company 16in upper.
