Reservations on buying a mini DisplayPort to VGA adapter
I have several aftermarket mini DisplayPort to VGA adapters (el-cheapo like Monoprice.com) to work perfectly with several Macs and various monitors and projectors.

Moreover, I am ever pleased and surprised at how well VGA works considering the old tech.

It should "just work" for a 20" VGA display. The Displays control panel will let you select resolutions up to the max supported by the display and extend the desktop from your MacBook. Performance should be excellent. 

I use a cheap mini DisplayPort adapter ($9) with 2008 and 2010 15" MacBook Pros and various VGA displays and projectors and it has always worked with no fuss, even over way-too-long VGA cables (50') connected to a high-def projector.

VGA is component video so signal integrity means a lot for image clarity. For a desktop 20" LCD display on a normal cable, it will appear fine.