I think that you should "retire" the CPU by letting it live out the rest of its useful life mining some crypto-currency.
Not really....
The SHA-256 ones are extremely difficult to profit from with anything other than specially designed hardware which is overpriced for the money you will get back (ASIC blockminers) ... this ship has sailed for the most part.
CPUs do extremely poorly at mining in general. If you are going to mine, mine the scrypt based ones with AMD gpus... but beware lots of people have thought of this already. Also the monetary values listed on the below website mean nothing when 66% of the mined blocks end up being orphaned for whatever reason, such as what seems to happen with goldcoin. If you thinking making 1 bitcoin in roughly 30-80 days (depending on the number of orphaned blocks) using 3 AMD R9 270x GPUs to mine around the clock is worth it... then by all means.

Keep in mind the difficulty will continue to rise as a bunch of people recently ordered AMD GPUs during the bubble when, at the peak, you could make a bit coin every 5-15 days.
On the other hand, if we happen to surpass those old market caps these alt coins briefly hit it would indeed be profitable to mine some of them here assuming you keep at it for a number of weeks/months... but not with ANY cpu. AMD GPUs only. Once you get the basics down and join a pool these miners pretty much run themselves with little supervision needed. My problem with crypto currencies is that anyone can create another one at any time, and who's to say which one deserves to be more valuable? Really the only value they have is in people's heads. A good speculative investment though. A good strategy is to try to find new ones right as they start up, ones that offer a lot of rewards per block right at the beginning. In the end I think the unregulated nature of it and human greed will cause the entire crypto currency thing to be nothing more than a fad because it's simply full of shysters and profit mongers right now.
http://www.coinwarz.com/cryptocurrency