Usually when I'm just cruising along or on the highway the temperature gauge is sitting right in the middle or right under it. However, the other day I noticed it was closer to the H on the gauge and when I would do a warm start it wouldn't go above 1k rpm and sounded like it wasn't going to turn over. I let it sit for a few hours and started it up again and it seemed fine. Did some city and highway driving and temperature returned to normal. Do I need to top off the coolant or am I overreacting?

 

You need to top off the coolant, and when you do make sure to turn the heater on full blast to get water into the heater core also.

 

How do I know when I've got enough or too much and what brand of coolant should I go with?

 

No more coolant will fit in the radiator and put any extra in the Coolant reservoir. I usually get my coolant from Nissan.