Is it possible for the expansion just to install the higher res options besides all the other new stuff? I want to fully play through IWD with 800*600 res first before getting into expansion's area and quests.

sure. the expansion (heart of winter) is accessed only by talking to this one guy holjider. when (if) you run into him just give himthe oh wait a sec i need to go get some equipment first i'll meet you back here.

then don't go back.

or if you do try the expansion, holjier will also bring you back when you want to.

There are a couple of ways you can play IWD / HOW. The one I recommend is to run through everything in IWD up to but not all the way through the ending of Lower Dorn's Deep and save there using something other than the quick save slot. Then go ahead and finish off IWD. Now go back, reload that save and instead of doing the final battle in Lower Dorn's deep (which isn't the ending of IWD) go back to Kuldahar and talk to Hjolldar, and start the HOW expansion. Once you get transported to Lonelywood, save again! Then you can finish off all of HOW, and be transported back to Kuldahar where you can then complete the original IWD.

The save in Lonelywood will allow give you a kickoff point to do the freebie dungeon Trials of the Loremaster!