Macys often has a $99 suit sale going - keep an eye on that, and get a couple of skirt suits in your basic colors. I like to stick with navy, black & grey -- if you go with brown, that's a whole other set of shoes & accessories. For basic button-up blouses, hit H&M. They usually have a good selection of solid colors, and they are sized (ie 2, 4, 6, 8) rather than S-M-L, so you should be able to get a good fit. Consignment shops can help too, but you have to be willing to put in the time to hunt.

The hardest thing about a formal office, I find, is getting over the desire to look on trend or to dress differently every day. Think of your clothes as a uniform (perhaps like the uniform you wore during your mustard days) - it's okay if you look exactly the same every day. Especially during the first few months, just swap out your blue blouse for white, your gray suit for black, and you'll be able to put together five outfits each week.

If you can get away with tights, do that as they last longer than hose. But, if you have to do hose, hit TJ Maxx -- you'll find a few basics there too if you hunt.

Keep an eye on Banana, Ann Taylor (regular & Loft) and J.Crew for sales on basics. Sign up for their emails so you know when the sales are - once you know your sizes, you can just shop online. Once you've got a workable wardrobe going, you can save your money for fun weekend clothes.

Congratulations on the big girl job!

Since you already got a great answer, I just wanted to add that in the August issue of Allure Magazine they've added a long article on dressing for the office including tips for looking chic and in style without looking frumpy or slutty! I found it informative and it might give you some shopping inspiration!

H&M has great prices on professional wear. You may have to sift through stuff but I've been able to find a lot of things there that look more expensive than they are.
Also, Value Village/Goodwill are great. I recently got Banana Republic and Caslon trousers for $5.99. The best stores tend to be in richer areas, so try out the Ballard, Bellevue, and Kirkland stores.

if you think the clothes at banana republic will do they email a lot of 25-40% off coupons. Like basically any day of the week you could prolly get something more than 25% off. Which makes me wonder if the original prices are set way artificially high.