A PC's idle temperature depends a lot on your case, everything in it from power supply to PCI-E cards and how it's all cooled. Personally, I wouldn't worry too much with a 45 degree idle temperature if your PC is built to be quiet. My CPU (Intel) is probably in the mid-high 50's in the summer half of the year under light/medium load. Yes, maybe a bit much, but it's quiet. \o/ The fans kicks in when I start gaming, but then I'm usually using headphones so that doesn't bother me.
You can probably lower your temperature a bit by tweaking your case cooling fans a bit using the motherboard vendor's utilities.
Heat isn't very likely to break your CPU, your case and CPU fans will spin up at one point and if that amount of cooling fails it will first throttle everything and if all other fails just hang until it's cool enough. Anyhoo, check that utility your motherboard manufacturer supplied to check what speed your fans are running. You can increase their speed a bit if you want a lower idle temperature. Lastly, obviously also check that you don't have too much of a cable mess inside your PC, to allow for good air flow.