On average you can expect a payout (a block rewarding you with 50 bitcoins) in about:
140 years, 25 weeks, 4 days, 3 hours, 30 minutes, and 17 seconds
And that's only if the difficulty didn't rise in that amount of time...oh, and blocks will not be paying out any reward by that time...oh, and you probably won't live that long
If you want to get into mining...acquire some top end AMD graphics cards (Radeon HD5970 or 6990 currently). They will very likely pay for themselves in a few months (look for some posts on how to correctly do the accounting for a mining business to get a true assessment of mining vs just buying bitcoins). Mining has been less profitable than just investing in bitcoins recently, but there's no guarantee that will continue...and we need more miners!!!