Obviously I'm a newbie overclocker (always used stock settings, stock fans), and am about to buy a new system:

Intel C2D e8400
4GB Corsair XMS2 DDR2-800
BFG 8800GTS (I'm assuming the double slot cooler will do a better job of cooling the rest of the case too in comparison to the 8800GT?)
2x120MM + 1x140MM Case fans (relatively low RPM ones, sadly)
2 hard drives, 2 cd drives.
Arctic Cooling Freezer 7 Pro

I'm still deciding on a heatsink/fan for the processor, but it's looking like the AC Freezer 7 Pro is a good one, especially at $20. How much overclocking could I get, with a decent safety margin? I'm not asking for something exact, but just to put it into perspective for me. To be honest, I have no idea it I will be able to hit 4.0GHz or 3.1Ghz. I'm still looking into overclocking atm. Thanks.

I'd assume you could do 3.6GHz with ease and still manage to get cool temps. I'd guess the max you could go would be 4GHz, but I think you'll be more voltage limited than heat, so give it a go.

how much power increase could I expect @ 3.6Ghz?

If you mean voltage, probably no increase at all.
I didn't need any more voltage with mine.

No, I mean total watts. Would OC'ing affect it? I will be using 2 8800GTS G92's with it (eventually), and obviously that will be tight.

Have you considered the Xigmatek s1283 CPU HSF? It's only $37 on newegg and is probably the best for the $ right now. The Thermaltake Ultra is slightly better, but cost about $15-25 more.
XIGMATEK HDT-S1283 120mm Rifle CPU Cooler - Retail

Artic Freezer 7s do not have a polished surface on the bottom. They are rough like sand paper. Alot of people have good results some dont. I was one that didnt. I will lap mine for my next build, because the stock cooler does better then my Artic Freezer on this build.

My AF7 was smooth and shinny out of the box. No complaints.

My guess is 20W increase under full load.