Shim are you going to use the stand for gigs or is it for a studio setting?
I purchased a the following item from Staples for use in my home studio. I usually have a Music Pad Pro and a 15 inch laptop on the top tier, it works very well. This can easiy accomodate a 20 inch iMac or the newer 21.5 inch iMac.
One of my boards sits on the desktop area. The desktop is large enough to fit a G70, Pa2xPro, Tyros 2 or any PSR series board. It also has a slide out drawer for your computer keyboard if needed. I chose not to install the computer keyboard drawer.
Here's the link : http://www.staples.com/Z-Line-Designs-Galaxy-Glass-Computer-Desk/product_580535?cmArea=page_rr