How to choke network preformance on OS X
Although you can use command-line tools to do this, Apple has a developer tool that allows you to do what you are talking about from a control panel. There are presets available, or you can set your own performance limits. The tool is called "Network Link Conditioner" and is available on Apple's developer download site with a package called "Hardware IO Tools for Xcode".

Here is a blog post discussing the tool (with screen shots):
http://mattgemmell.com/2011/07/25/network-link-conditioner-in-lion/