dobi really need any extra coolers

With your summer temps, it couldn't hurt to throw some more fans in there and maybe look at a 3rd party CPU heatsink/fan.

Looks like your case will allow for two more 120mm case fans to be put in. One more front mounted intake fan and one side mounted intake fan. The side intake will be simple to put on, the extra front intake might take a bit more effort to do.

Your motherboard only has three 4pin fan connectors so if you get more fans you'll probably want to make sure they have a molex connector so they can get power straight from your PSU. Any 120mm case fan will do the trick really, so just look at what your parts retailer has for 120mm Case Fans. Antec of course makes some so when in doubt, go with the same manufacture as your case. This will also be the cheapest way to increase airflow in your computer.

You're running a stock cooler for your CPU. If you're concerned about CPU heat, you may want to look in to an aftermarket cooler for AM3+ sockets. This will be the more expensive route to take. I'd go with adding case fans first and watch your temperatures (your motherboard should come with software to monitor temps). If your CPU is constantly getting close to, or exceeding temps then your going to want to put a 3rd party cooler on. For air cooling, the heatsink/fans can get pretty massive so make sure you pickup one that either provides a back plate or check to make sure your motherboard has one. Heatsink/fans like the Noctua NH-D14 expect you to check as they don't include one and also tell you how to check to make sure your case can meet the size requirements. If you go water cooling, the size is much smaller but the specific cooler may have similar requirements of back plates or mounts. Just make sure to fully understand how an aftermarket cooler is going to attach to your computer before you buy it.

Your PSU is just barely over the minimum requirement for your video card, so if you're throwing extra fans and are at all considering water cooling (not a bad idea given your area) then you may want to consider jumping up to a higher wattage PSU. A 650watt would likely see you through any power requirements if your not going to run a dual videocard setup.

As for what overheating does, well simply put it makes things stop working correctly. It can be as minor as a little glitch or as major as a component just out right dying. All depends on what overheats and by how much. Your case is pretty big, and if you throw some extra fans in there you likely wont have to worry too much. In the summer months keep an eye on your temps. If you're still going too high then a 3rd party CPU cooler may be needed.