This is what I did with my old tower. The wife likes to watch her not-so-above-the-table TV show streams on it. It is an integral part of our cord-cutting home entertainment package.
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Backup servers, Secondary Video Capture/Fraps machine, and Enterainment Center are excellent uses.
My son does a lot of programming. Mostly 3D engine lately. He just started on a Minecraft clone.
Anyway, he is on a mac mini right now. Good machine for just about everything except for what he wants to do with it.
He does do IPhone/IOS development too which is why he has the mini in the first place.
I was going to piece together a simple gaming rig for him. Since he doesn't do much boxing yet he doesn't need to much power right now.
If you make it an HTPC you can use it as a server for all your dvd's and videos. Plus you can put all your music on it and play it from your tv. Browsing the net from the tv is great. I turned my old boxing machine into an HTPC and love it.
http://www.dual-boxing.com/showthread.php?t=45299 Check it out. System specs are very similar to yours.
I kept mine, and I'm glad I did. I have it hooked up to my old monitors, old keyboard etc etc My motherboard died recently, so it got some use while I fixed my main rig. Hard to believe it is already so out dated, yet still is pretty fast for most tasks. Q6600 has some staying power for sure.
I plan on upgrading my x58 as soon as the x79 boards are a bit more mature, so the trickle down effect will be in full force.