I have tons of CPU and a reasonably good GPU.
Should I used the CPU to mine namecoin and the GPU to mine litecoin? Or perhaps some other combination? Or what?
You can try to mine Novacoin which is maybe less difficult to mine, but be careful, this money is 'a sacm' for certain people (the creator has pre-mined the money).
You also have the TerraCoin, the NameCoin or the PPCoin.
Check out http://cryptocur.com/ to see how to mine these. Coinotron is also a great pool to try to mine alt money.
Namecoin is mergemined with Bitcoin, so the difficulty is near as high as bitcoin. You may as well mine bitcoin and connect to a pool that allows Merged Mining Namecoins -- and you'll get uBTC bitcoins with no additional effort.
I recommend to leave the CPU alone especially if you're on windows, because i've noticed mining with both GPU and CPU is terrible because the CPU is required to submit and if the CPU is tied up with another application -- say NMC mining then the KH/s from litecoin will go down.
TestNet Bitcoins - the chain is usually at difficulty 1, so one can mine it reasonably well with a CPU. However, the coins themselves are worthless - it's just a system for testing your Bitcoin solutions.