Thank you Vinton Harper! It was driving me CRAZY trying to remember his name!!

And yes, you and Lovehair are right, he's posted some GREAT pictures and anyone who wants to see them can now search his user name and find them.
I think I'm right that its a fullcap or at least a 3/4 cap?

I don't think they are 'curtains', I think they're temple flaps.
But a while back during a discussion about wide foreheads and using temple flaps, I used Mr Burns picture above to show exactly what you are referring to Lovehair.
I removed his temple flaps and kept the straight hairline and immediately it look SO wrong.

BD, Lovehair has given you a very good link to American Jambo's thread.
He's a perfect example of how doing research and getting your template right REALLY pays off.
Its not often that someone can get such great results on their first order.
Most of us got it wrong at our first attempts.
But the best bet is to remember that over time, you'll buy and wear several systems and each time you can make alterations and adjustments and try different bases.
Those changes will only be noticeable to you and make wearing better for you.
So its getting a balance between understanding the options and not getting too bogged down by them.
Easy for me to say, I know!

The poly perimeter thing is a difficult one to answer.
I think whether you have or need one comes down to how you think you're going to attach the system and whether it would concern you if you or someone could feel the edge of the system if they were to run their hands from your natural hair to your system hair.
A poly perimeter is useful if you want to only attach your system around the edges using tape or glue. It prevents adhesive residue that breaks down and gets gooey from working its way into the lace and creating a mess. Its much easier to clean than just lace.

In my opinion, every time someone chooses the 'easy' option or something that makes a system more durable, they compromise to a degree on detectability.
So in the case of a poly perimeter, whilst it won't be visibly detectable, as I've said, you'd probable find it detectable in terms of being able to feel it.
But a lot depends on what works for you as an individual. So if having a perimeter makes wearing better and easier for you and the 'feel' factor isn't a concern, then thats the right choice for you.
If you decide you don't like it, you can always change it on your next order.

To me, the key things to concentrate on are colour match, hair type (curl and shaft thickness), base shape and density.
If you get those things right, you're 90% there.
The other 10% are things like knotting, attachment, styling, cut etc.
Read American Jambo's threads because he's documented his whole process and its full of useful information.
Good luck!