A rule of thumb to determine how many DMD you will mine in a day:
15 divided by the averaged diff, multiplied by the MH of your rig.
Ex: 15 / 40 * 80 MH = 30 DMD per day.
EDIT: If you are mining 24/7 for a week, it makes no difference, whatsoever, if you are mining solo or in a pool - it is like a lottery - pure statistics/probabilities - you WILL get the results above, on average, after a week. It doesn't matter how small or big your are - this is a misconception that small miners are disadvantaged... If you have a good internet connection, you will reach those numbers.
But, if you don't mine constantly, or, if you are a coin hopper, then, yes, it will make a difference.
Solo mining promotes loyalty to a coin - Pools promote hopping between coins.
EDIT2: BTW GRS (or any other Groestl-based coin) is the same formulae above, just multiply the result by the reward value (currently at 353 GRS/block).
The statistics part of this is more or less correct. However, pools to not exists only to concentrate hash power -- at least not the classic pools. This is more a feature of the p2pool type pools, that do indeed promote hopping.
A "normal" mining pool is more of a coin network service. It not only provides for more even earnings, but also additional services, such as worker monitoring, various statistics, communication etc.
A coin can survive without any working pool, everything solo mined. But, imagine someone invests in huge hash power and is able to solo mine most of the coins. When you don't find a single diamond in a week or a month, you will eventually give up and that guy will end up with all the coins. It's statistics again...
In situations like this, you could look at pools as the nuclear weapons in the Cold War era
Anyway, again: look at pools as service to miners. This is what they do best. The small payments is secondary.
