Lightyear69,
It used to be that computers were used to mine bitcoins, but now that's not done anymore. Those were the days of a couple of years ago.
Up until maybe August of 2013, you could still use graphics cards to mine, but those took a huge amount of power, and they became not profitable once ASICs (application specific integrated circuits) came out.
You can still use computers and GPUs (graphics cards) to mine what are called scrypt based miners, like Litecoin and other scrypt based coins, but you need pretty powerful graphics card, like 4-5 if you want to get anywhere with that, and a beefy power supply to boot.
As an example, I run 2x7950's in one computer for Litecoin mining (get about 1.1 Mh/s with both, it's a small setup with a 750 W power supply). You can't really put more than 2 of those cards with the case closed, because of the heat they generate and dissipate (200 Watts each, 400 watts total power to dissipate).
Most people who run many (4-5) graphics cards have open air rigs with powerful fans blowing on them and increased spacing between graphics cards so they do not burn each other up. I have not personally gone that route, mainly due to space and safety (kid) concerns.
As for bitcoin mining, to get to 400-600 Gh/s, you need some powerful miners, like several AntMiners, or a bunch of Singles, or a few newer Avalons, and burly power supplies.
My recommendation would be to start small, do not burn a lot of money, and configure any existing hardware you have to mine some litecoins, or maybe get a cheap ASIC off E-bay. Some of the older generation ASICs have come down in price. (like maybe a Jalepeno or Little Single). Or, get a redfury USB miner and run that for a bit.
Hope this helps.
As I mentioned that the big plus for me is that i need not to worry about electricity bill.
You rightly said and I agree that its better starting slow and I would be content to make anything about $500 or more/month. Having said that, At the moment I am only interested to mine through using computer only.
As you said-
I run 2x7950's in one computer for Litecoin mining (get about 1.1 Mh/s with both, it's a small setup with a 750 W power supply). You can't really put more than 2 of those cards with the case closed, because of the heat they generate and dissipate (200 Watts each, 400 watts total power to dissipate).How much $ one will be able to mine having above set up i.e. 1.1 Mh/s?
Your piece of writing really very helpful, Thank a ton for that!