Pools hashrate does not matter, really.
The higher it is, the smoother your earnings will be, but its either bigger parts of fewer blocks or smaller parts of more blocks, average the same, as long as the difficulty stays the same.
Exactly

Your hash rate should be stable - it will fluctuate slightly depending on individual worker luck - but if you had about 2 GH/s last week, then you should still have 2 GH/s this week. This is regardless of difficulty increases, increases in the pool hash rate or increases in the network hash rate.
That is the easy bit - your own hash rate is a relative constant.
Now, assuming your own hash rate remains constant (and you are not adding in lots of usb miners - so you stay at 2 GH/s):
1) If the pool hash rate increases but the total network hash rate stays the same (so in other words, other existing miners move their rigs over to slush's pool)?
Then you should expect to find blocks more frequently - but your share of the payouts will go down slightly. Net effect is you earn the same over a week.
2) If slush's pool hash rate remains the same but the overall network hash rate increases?
Then you should expect to find blocks less frequently - but your share of the payouts will remain the same. Net effect is you earn less per week.
3) More typically, if slush's pool hash rate increases and the total network hash rate increases proportionally?
Then you should expect to find blocks relatively consistently - but your share of the payouts will go down. Net effect is you earn less per week.