Worried about the overheating

Hey guys. I'm getting an Xbox 360 Pro package this Christmas, but it's not the new model with the HDMI port and the new heatsink. Instead, it's a model that apparantly was shipped one month before they upgraded it. I'm incredibly worried that my Xbox will overheat while I'm playing. I do have a fan that I'll use to keep it cool, but still...

Could you guys post how long you've had your Xboxes? If you had any probs? How it's set up? How long I can play for without fearing the overheat?

Thanks

i have had mine 4 around 8 months and i have it horizontle in a open area. just evey hour or a half hour touch the back of the xbox and if its hot turn it off but if its warm dont worry about it.