My SS pot is 5 gal.

I plan on picking up an Al pot this weekend (probably about a 6 gal or so).

My other limitation is my gas stove. I think 4 gallons is at the upper limit of my boiling abilities.

First, I'd suggest that you mash as much grain as possible....I can mash 7 lbs in a 4 gallon pot using 1.25 qts water/lb for both the mash and rinse/sparge, so you should be able to get away with more. Don't worry if you end up with more wort than you can boil (which you probably won't). You can just boil it in the second pot with some more water and add your extract there, which is what I'd recommend. Once this comes to a boil, you can add you extract so that your total gravity ends up where you want it for the batch.

The best way to think of this is in total gravity units for your batch. Once you do your mash and sparge, figure out how many gallons of wort you have and figure out the specific gravity. Multiply those together and you've got the total gravity units from your mash. Let's just say you've got 4 gallons of 1.050 wort after the mash, that's 200 gravity units (4 gal X 50...just drop the 1.0 from 1.050 for this calculation and subsequent ones). For a 5 gallon batch of 1.090, you ultimately need 450 gravity units (5X90). So you'll have to add gravity units using your extract. If you're using dry extract, it gives you 45 units per pound, so you need 5.6 pounds (250/45) to hit your 1.090 for your final batch.

That make sense?? If you want to hit your gravities and especially if you have no idea what your efficiency is going to be, you really need to be able to work with this concept on your batches. It should allow you to hit your gravity within a point every time.