Looks like the next version (4) of google earth has just been released in Beta. you can download it here I quite like the look of the new interface, it seems much less clunky than the previous one, looks a lot more proffesional.

It also seems to have something called Smart KML which allows KML to be downloaded in bits. Looking at the new KML 2.1 spec it seems to be a scale threshold, you can split you KML into regions based on a geographic area and screen scale. these become active with the view at different scales it should help KML to scale a bit better than it does at the moment when you have to download the whole file in one go.