1) 49.0 Mhash/s at current difficulty will produce about ฿0.001 per 24 hours of mining, or $2.7 a month. Given that you most probably mine with a GPU, you will waste more energy and GPU lifetime with that.
2) Bitcoin wiki (
https://bitcoin.it/) has articles on anything.
Mhash/s is megahash per second, used to measure mining speed. During mining, the miner calculates the SHA-256 hash of a block, then adds a nonce to the block data, with is a number starting from 1, then checks if the hash is less than a certain number set by current network difficulty, increases the nonce than starts again until the hash meeting the requirements has been received (SHA-256 hashing is made in such a way that if you make a slightest addition to the data, you get a completely different hash, so you cannot predict it until you calculate it) and then the block is created and propagated across the network. The more hashes are generated per second, the more likely it is to solve the block, but everything is probabilistic.
BTC means BiTCoin, the name of the currency
3) Very probably so.
4) Official client although it verifies the complete block chain every time you start it and thus loads for several minutes on a cheap notebook. For mining, maybe cgminer.
