You can try Ericom Blaze, a software-based RDP acceleration and compression product that accelerates RDP performance by up to 25 times and delivers higher frame rates and reduces screen freezes and choppiness.

Ericom Blaze works with any standard RDP host, including VDI, Terminal Servers and remote physical machines.

You can read more about Blaze and download a free evaluation at: http://www.ericom.com/ericom_blaze.asp?URL_ID=708

You can also check out a demo of someone using Blaze to connect from a Mac to a Windows PC:
http://ericomguy.blogspot.com/2010/12/video-of-ericom-blaze-on-mac.html?URL_ID=708