Spend the money and get your Aeron, or something of equal quality. I use the Steelcase Think. Your chair is one of the biggest factors in how many hours you are able to work, how well or poorly your back is aligned, and how comfortable you feel. Do -not- skimp on your chair; trim down spending on building your computer or on that second display if you need to.

If you're serious about your work, spending as much on a good chair as you do on your computer (usually less; a quarter as much for me) isn't begging too much. you can make the money back with the longer hours it lets you sit comfortably for and in the hospital bills you'll avoid from having a properly aligned back. It's also important to lift weights daily if you're going to be sitting in a chair all day working. It'll fix any adverse effects sitting from extended periods has and give you energy.

If you have trouble finding the money to afford the necessities for your work, I am always asked by clients to refer them to coders, and there are many freelance gigs floating around out there if you look.