Need advice on what to buy.

A dell or a hp if you dont want to assmeble it

You're going to have a hard time finding anything for 300. Set your sites more realistically, like at $550

Theres a pre-build subsection over in homebuild, maybe there would be a better place to ask,
but your restrictions of NEW and $300 dont indicate a high level gaming pc to me, as abekl says, maybe wait till you can up your budget if you insist on a pre-built unit
Moto

You could maybe get a mid range gaming pc for 300 if you built it. Otherwise, fork over more money.