im upgrading from an Ibanez gsr200 that ive owned for about 1 year now, i started playing guitar first so bass was fairly easy to pick up on. anyway its a decent bass but im looking for something in the 350-650 dollar range, im a fan of thin necks, im considering a fender or a schecter but i dont know too much about bass guitar companies and what the good brands are.

im into hardcore music like Underoath, The Used, and Haste the day.

any suggestions?
You have quite a nice range for what you are willing to spend.
For what you listen to, I would hav to suggest you take a look at Schecters, and for extreme speed, Ibanez's. Particularly look at the SR series in Ibanez, and look at the stillettos and hellraisers on schecter.
Do you want 4 or 5 strings?
Quote by Azgirio
Yeah, you definitely raped his churches and burned his women.

Quote by LordBishek
Forgive the bluntness, but what in the chucklefucking hell is this?
Schecter basses are quite nice. I have a Studio 4 and have no major complaints about it. If you're a guitar player doubling on bass then it is probably all you'll need. Ibanez is also good. But keep in mind if I had to own one instrument I would choose one with a 2 octave neck over a 20 fret neck. That alone takes most Fenders out of the picture for me, even though they are nice instruments.
underoath and the used are not hardcore.

underoath is post-screamo
the used is emo
haste the day is...eh...borderline hardcore.

anyway i got an Ibanez SDGR 6 string bass and the neck is thinner than the guitar i have.

http://bass-guitars.musiciansfriend.com/product/Ibanez-SR506-Soundgear-6String?sku=515058 that is the bass i own and it is beautiful. didnt need to be set up at all or anything.

i think they make a similar 4 string version. either way ibanez is a great brand, especially for the price range.
Ibanez S7521qm 7 string
Ibanez S771pb
Fender Jaguar HH Special
PRS SE Custom 24 7 string
Epiphone Les Paul Custom
Im refering to the first album by the used, the one that had meaningful lyrics, and by hardcore i mean fast paced but not so fast as metal, more brutal.

but i dont really believe in labeling a bands style,
*facepalm* why would you even try to call the used hardcore?

try ibanez basses
Ibanez S7521qm 7 string
Ibanez S771pb
Fender Jaguar HH Special
PRS SE Custom 24 7 string
Epiphone Les Paul Custom