What to do with outdated but snappy PC?

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Yea get it as low power consuming as possible and make it a file server/downloading machine.

I also have a Q6600, less ram and 2 x 500 gig drives, I have a reasonable (but not great graphics card). I manage OK although more ram would be a boost. If I were you, I'd pack the desktop PC away until you've had a good trial with your new laptop to access is suitability for the job.

Once you're sure about being happy with the laptop, you could sell your desktop on Ebay, buyer collect. Sell it for bits? Or give it away on a site like Freecycle.

I sold my Q6600 to a friend cheap, his kids use it and it's really good for that/them. It was still pretty ok, snappy enough, just not so fast on the batch processing.

Craigslist?

donate it to charity.

Put an SSD in it and youll be amazed. Just avoid OCZ, spend a bit more and get a good brand like Intel, Samsung or Plextor (current ones are getting very good reviews) or a few others. OCZ = fail.