If you only have 100GHz and you're running it on your video card, you may or may not break even on profit. But, that's up to you to decide.
BTC, Bitcoin, is here to stay. It's fluctuating a lot since it's new, and I embrace it.
NMC, Namecoin, is niche. Unless a large DNS ban/block/hijack happens, it will never take off.
IXC + DVC are just alternate coins, and no huge value in general.
LTC, Litecoin, is the other 'big' one to look at.
Litecoin + Bitcoin will be the 'big players' in digital currency. Everything else has a slight chance, or is too niche for anyone to care. IMO, don't bother with anything besides Bitcoin and Litecoin.
Also to watch, is PPC, Peercoin. You don't have to mine it like other coins, you can actually create more coins just by having coins.
The biggest turnoff and barrier to newbies for Bitcoin is security. Bitcoin protocol itself is very secure. However, your coins are stored in a .dat file. If someone swipes that file, you're SOL. If someone keyloggers your password, you're SOL. If you make a typo in the address you're sending to, you're SOL.
There are many ways to get around that, but they are barriers to widespread use.