I have to choose between these two processors AMD ATHLON 64 1640 2.6 GHZ AM2 or INTEL CELERON DC 2.2 GHZ E1500 , the processor will be used in a machine that has poor ventilation and will be powered on between 5 to 8 hours, the pc will be playing videos and music all the time, at least 150 Gb of multimedia files, which one will have the best performance with 1 gb of RAM?
Celerons are notoriously known for their slow performance so I guess go with the Athlon 64 
I am a huge fan of AMD and I am currently using an AMD Athlon x2 dual core 64bit 4400+ Processor. I have been using them for sometime now and I have had no problems or complaints. Runs great.
I have my computer running 24/7 and my old computer with an AMD Athlon 2800+ is my gateway/server and is always running without a reboot.
IMO:I don’t use Intel anymore, I am not that crazy about them. But if I was to use one of their processors I would not use a Celeron at all.
Why poor ventilation? If it is poor can’t you throw another fan on it?
for what you say your doing it will not matter. the amd might run a little cooler. I’m using an old P4@2.8Ghz (socket 478) with 768Mb of ram and it’s fine for music and videos. the single core amd can get the job done and is about $10.00 cheaper.