Doors and Windows - Replacing a casement window

I have a few questions about replacing a casement window.

The measurements are not exact because I am at work and have only just found this forum. I hope the figures are close enough for some help.

1. Currently I have a old wood window in my kitchen and would like to replace it with an aluminum clad. The framing around the current window measures about 5.25 inches deep and fits in nicely so that it butts up against the inside drywall and comes even with the siding on the outside so that the exterior moldings fit snugly against the framing of the window.

2. What kind of replacement window do I need to purchase? Do I need to buy a window for a 4 inch wall or a 6 inch wall?

3. The current window does not fit snuggly into the rough opening. there is a decent amount of room on the top, left and right sides.

4. When I get the replacement window, do I need to frame it with wood like the old window or should I be able to place it in the rough opening and screw it into place?

5. How do I determine the size of my exterior wall? I know that there are 4 and 6 inch walls.

Thanks for any help.

http://www.hometime.com/projects/howto/windoor/pc2wd04.htm

http://www.hammerzone.com/archives/window/replace1/oldhome/anderstw1.html

http://www.hammerzone.com/archives/window/replace1/oldhome/andersxl1.html

http://www.handymanusa.com/questions/windowreplaceq.html

http://www.homestore.com/HomeGarden/HomeImprovement/HowTos/HowTos/DefaultWindows.asp

The best I have seen online for window installation, great attention to detail !!