I purchased this USB to VGA device, with the goal of extending my laptop to support two external monitors (instead of one):

SIIG USB 2.0 to VGA PRO (JU-VG0012-S1)
http://www.siig.com/usb-2-0-to-vga-pro.html

I think this device has its own video card built into it, and I was expecting it to perform independ of my Laptops internal video card, in that, I was expecting to be able to run my current 2 monitors at the same resolution I usually do, while adding a third monitor with this independent device.

Here's the resolution I run my two monitors with:

1440 x 900 - Laptop's Native Monitory
1920 x 1080 - Laptop's HDMI Port's External Monitor

However, after adding the SIIG USB 2.0 to VGA PRO (JU-VG0012-S1) and a 3rd Monitor, this is the highest resolution I was able to achieve on all three monitors together:

1440 x 900 - Laptop's Native Monitory
1440 x 900 - Laptop's HDMI Port's External Monitor
1152 x 864 - SIIG USB 2.0 to VGA Pro Monitor

As you can see, in order to run all three I had to reduce my HDMI monitor down from 1920 x 1080 to 1440 x 900. Otherwise, it would turn off.

I thought this device had its own ability to power resolution to this 3rd monitor and I thought it wouldn't adversely effect the resolution of the other monitors that are powered by my laptop's internal video card. Yet, as I've said, I have to reduce the resolution of my current monitor setup to acheived the addtion of a 3rd monitor via the SIIG USB 2.0 to VGA Pro.

For more details about my hardware and configurations, go here:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1227833