I have just finished building my new gaming pc and i'm using a i7-870. I have been reading up on the i7's and one common concern that keeps popping up is that they have a tendency to run hot and that the stock heat sink/fan is not that great. Is this something that needs to be replaced right away? or is it just something to keep an eye on? thanks.
If you're not overclocking then you needn't replace the stock HSF unit unless you have particular issues with your temperatures due to your location, ambient temps etc. Keep an eye on your temps and see how they hold up, but as long as they are within comfortable limits at idle and full load I wouldn't worry about replacing the stock HSF. If you do overclock at any point then obviously it's a different story.
HWMonitor from CPUID is a good utility for monitoring all of your hardware and will also show you the temperatures of most of your components: