I just built my own pc a week ago, its a mini itx in a master cooler elite 120 case. I have an i5 3570k with stock cooler and a HD 7850 Gigabyte gpu.
At the current moment, my system sees idle temps in the mid 30's, when I open firefox it stays in the low 40's. when playing games for hours it ranges from high 50's-to high 60's. In Prime95, a 30 min blend test gives me temps in the high 80's, almost hitting 90.
My plan for the PC is that I would leave it on all the time, and use it for bouts of heavy gaming and browsing. I am not going to overclock. That being said, are these temps okay for what I want to do and is my computer going to suffer because my temps are too high? I just feel like I spent so much money building it, Id hate to see its potential or lifespan limited. Also, Id rather not opt for an aftermarket cooler unless it would deffinatley increase the lifespan of the pc.
Those are normal temps. Prime95 stress your CPU high but you'll never see those temps in any real world usage. If you do like to buy an aftermarket CPU cooler (which is unnecessary unless you want to overclock) however, the coolers I would suggest are the Cooler Master Hyper TX3 and Hyper 212+/EVO. Don't go any higher than that.
You may be able to undervolt your CPU depepnding if your motherboard allows you to do so. That will bring the temps down as extra voltage=heat. Most motherboads have strange voltages and set them too high. This is ideal if you arn't going to overclock. But in my personal opinion about the cooler, I would still bag an after market cooler as the stock ones are rather nasty.
I would recommend getting yourself a decent CPU cooler, that would certainly help lower the temps.
I recommend the Scythe Big Shuriken, good low profile to fit the case and much better cooling power than stock.
There is also an additional fan slot for your case 120mm that you may consider getting to help improve cooling.
Edit: Just seen other people have recommended coolers, just be careful on what you select as this case can only accommodate coolers up to 65mm in height due to the PSU sitting above the CPU. That is why i suggested the shuriken at 58mm height.
Those temps seem about right for a stock cooler and a ~20°C ambient room temperature.
The cooler you can keep your chip, the longer a life it will have. That being said, using the stock cooler the CPU will be long obsolete before it actually fails on you due to heat stress. If the temps bother you, throw a 212 EVO on it and be happy.
Also you have an overclocking chip (and I assume a Z77 board), why not overclock? A basic overclock like 4.2Ghz is a very nice performance boost, doesnt put out much more heat and requires little skill. Its just changing a number in the BIOS and the mobo will automatically figure out voltage.
I would also recommend an aftermarket cooler. Even with those temps being artificial worst-case, you still have the potential for a lot of heat being generated in a small chassis. Have a look at the Coolermaster GeminII M4. I've got this cooler myself, on my HTPC, and it's very quiet. It's also low-profile...your chassis can only take a cooler up to 65mm in height, according to the Coolermaster website so don't go for the coolers EzioAs suggests as they won't fit in your case!
Edit: I meant to add that the Scythe Shuriken isn't a good bet either. I'm sure it's a great cooler, but it uses the same push-pin style mounting arrangement that stock heatsinks do. With the low profile of the cooler, and depending upon the arrangement of motherboard heatsinks around your CPU, you may find it impossible to mount correctly. The GeminII has a backplate mounting arrangement and is much easier to install (I've tried both).