Stay away from USB mics. USB mics have to package an audio interface, pre-amp and microphone in to one package. It limits your ability to upgrade and is more expensive in the long run as it limits your options.
Instead, get a decent quality audio interface (either with a built in pre-amp or a separate pre-amp) and get a separate mic. This will allow for you buy multiple distinct mics for different purposes in the future without having to buy multiple sets of audio interface and preamp hardware. It also allows you to upgrade quality incrementally in the future with fewer changes to the overall system.
$300 is a pretty limited budget for really good quality recording. You are probably looking at a cheap audio interface like an M-Audio or similar and a cheap, workhorse dynamic mic like an SM58 or possibly a dirt cheap large diaphragm condenser.
You really don't have enough budget to be too choosy about mic characteristics, but rather will just be looking for something that actually gets passable sound in that price range. Most really good large diaphragm condensers for that kind of thing are more in the $300 to $600+ range for the mic alone, but you need an interface and a mic in that same price.