Thanks, I did get it to work eventually. Here's what I did, though just using conditionals like you suggested would probably be better.
/* nav width */
#access {width: 735px;}
/******* searchbar in menu **********/
#header #search{
width: 200px;
height: 30px;
background-color: #000;
float: right;
padding-top:8px;
padding-right: 5px;
}
Moved the search code in header.php above the access div. Changed the #access width, then added a width and height for the search so it would drop down into it.
I'd need to change #search background color if I want to change #access background color, but it seems to work ok. I'll look into using conditionals, I've never used them before.