as for the icons i just had that link. the same icons are existing as regular images that you could use.
i've had a quite extensive research on the base64 stuff for a couple of weeks for a client.
from my point i can confirm that base64 encoded data uri's are fully supported starting IE7...
well, if you want to know, you even could double encode the contents in all modern versions and browsers.
besides that... do they rounded corners and gradients and all other fancy stuff work in ie6 anyway? no...
Added 3 minutes later:
also the icons in buttons seem a bit missaligned. maybe you can dead center them?
Added 49 seconds later:
also maybe add different cursors to disabled elements
Added 1 minute later:
maybe add flat tooltips. the user maybe gets confused with the button-looking tooltips you have now.