Jcrew and/or BR are the places to go if you want to look the part of the young, stylish, polished executive. Both have really nice suiting selections - investment pieces. Ann Taylor is another place you might try as well, although their clothes are aimed at a slightly older crowd.
One thing nobody mentioned is shoes and purses. As an executive, I recommend you invest in good quality for these, especially the shoes. It's a subtle but important detail, I think. A great shoe and quality purse can up level any outfit, and also add in some personal style to more classic basic pieces.
Likewise allocate a portion of your budget for scarves, necklaces, a great watch and other accessories. These don't necessarily have to be top dollar but they add a lot of personality and style.
Forgive me if you already know this, but I'd take care to dress modestly. Camis or higher necklines to cover cleavage, skirts at the knee and not too short, closed-toed shoes that aren't too high/too sexy. I think this is wise in the work force no matter what your age, but as a 26 year old executive, it's even more important.
How about your hair? A great haircut can go a long, long way towards up-leveling one's overall look. Or wearing long hair up in a nice, neat chignon. It's these little details that make a big difference, and which I tend to notice on our executives.
I also think that erring on the side of slightly dressier than everyone else is just fine. When I was your age, in my first management role, I looked about 13 (I'm very short and have a baby face) so I would dress up more formally than everyone else. If I didn't, I'd get confused for the secretary (this was back when we had secretaries, not admins!)