GPS Review for Garmin, Magellan, and Tomtom

I got the GPS as a gift. It comes with a mount already. Why would I buy a separate dashboard mount (available on Amazon for $30)?
Also, I’ve seen antiglare screens. Are they useful? Will they help reduce washout that can occur when the sun shines on the unit?
Thans for your help!

I do not need a fancey one I just need one to punch in the courdinates and be able to quide me.

I marked several places on my Garmen GPS when I was in the field on my last trip. When I download my GPS coordinates that I programed into my Garmen hand held unit, the all seem to be off by quite a few meters. When I plug the same units into google earth, they all show up in the wrong place, but my GPS can find the coordinates time after time when in the field.
Are there different systems for calculating GPS that could be accounting for this? If so, is there a simple formula to correct for this? Or is there something wrong with my GPS?

Hi, yes you can:
CompUSA stores:———————————…http://www.compusa.com/products/products…
Target Stores:———————————…http://www.target.com/gp/browse.html/ref…
Radio Shack Stores:———————————…http://www.radioshack.com/search/index.j…

i have a garmin 76 and want to download a bluechart map. i had to buy an adapter for the cord they sent me so it would plug into usb, but my computer doesnt recognize that its plugged in. Any advice????