I want to take out the 728X90 Adsense banner below my logo at top of my page (see my signature). I go to APPEARANCE>EDITOR>HEADER.PHP and I can highlight and delete all of the specific code. But my question is the <div class="banner"> just above the code. There is also </div></group> just below that adsense code. Should I leave those in? I ask, because I want to replace the 728X90 ad with a link unit sized 728X15
The AdSense code is encased by the "banner" div. When you replace the banner code, you might want to include the "banner" div if it handles layout properly for the new-sized ad block. Or you can take it out and define your own, whatever suits your purposes. I suspect it does stuff like centering the ad block (though I didn't check), in which case you'll want to leave it. The AdSense code shouldn't be in the hgroup, and W3 is removing hgroup from the HTML5 spec, so you can bin those tags. Just leave the h1 and h2 lines as normal.
Actually I am planning on inserting a 728X15 text link unit. Should I paste that new unit like this: <div class="banner">LINK UNIT CODE HERE</div></hgroup> ???
Assuming you mean an AdSense ad unit, yeah, they all work pretty much the same. You'd just replace the existing code with the new one. The CSS might do something weird, but hopefully not (that would depend on exactly how AdSense implements the unit and so on). You can get rid of the <hgroup> and </hgroup> tags: they don't do anything.
TIEro noticed all. However if you class banner do anything with css, i mean if it set fixed the height with css then you may need to edit css too for this class.