NO! Sometimes for poster work 200 dpi is enough, but never that low as 100. The exeption would be if the end result gets printed on some inkjet system, which can yield pretty good results from 100 dpi. 300 dpi is what you need for sharp reproduction in print. In pixels, for a 11"x17" image that means 3300 x 5100 pixels in document size. The 'resolution' is about how much pixel information goes into one square inch. For print that has to be much higher than for monitor reproduction. Because you mention the extremely large brush, I remember a mistake I made a couple of time. Look if you, accidentally, did not set the resolution in pixels per cm. That's a whole lot more than per inch... Photoshop can never have enough RAM, but the processor speed of your PC is equally important when working with this serious image sizes. Documents can easily need more than 1 Gigibyte of space (five times the document size in hard disc space), so you have to have that too. That's why Photoshop professionals always want bigger, better, faster machines. It is never enough... Good luck!