Your post wasn't very clear. That is probably why no one responded but I will try to help. If 2x6x2 means you are going to break the ten feet of window into a mulled window of two foot flankers and a six foot picture window, that is very common. As for your question about making the room warmer, an upgrade to low e IG will help immensely. Being that the window is on the southside, you may also think about passive solar with the window as long as you have some protection (shade trees or large over hang)in the warmer months. Cardinal's 179 is excellent for that. If you don't have any protection, I would recommend the 366 product. Hope this helps.

house is minimally insulated. 2x4 walls and 2x6 rafters overhead with R-19. cannot get into overhead space unless i got thru roof vent. no closet roof access on this end of house. multi-level. so the house shell has weak insulation. will installing a better R value window with better infiltration specs improve the rooms heat loss? IG, is the a gas filled term?

Means Insulated Glass, and a better R value in a window with an poor insulated wall is kind of like putting a storm door on a screened in porch.....Not going to do much!!!!