[mikeJ]

You flop trips about 1 in 75 times, you flop two pair about 1 in 50. Total ~= 1 in 30, you'll flop two pair/trips.

You'll flop a flush draw OR 8 out straight draw about 15% of the time (this includes paired flops)

You'll flop a four flush AND a 3 out or 6 out straight draw about 2.6% of the time.

So basically, on ~5% of flops, you'll be happy to get it in. (2.6 + 3.33 ~= 5)

These numbers are taken from Mike Petriv's "Hold'em's Odd(s) Book". And, no, I'm not a book salesman.

Also, I just call preflop. Unless you are trying to get people to loosen up to your reraises or they will call reraises wide then c/f on lots of flop, I think you're better off trying to hit a big flop and win a big pot.