I have a GIGABYTE R9 290X with 6+8 pin power connectors, so it can deliver 300 watts max. Is ok to increase the power limit in MSI Afterburner and what would happen if I reach the limit?
Raising the power limit allows your card to draw more wattage. Reaching the limit would be fine (assuming your PSU can handle it), because that's the whole point of raising the power limit.