Thank you for your suggestions. They are taken into consideration, and the UI is just a preliminary one since it will of course be improved and will ask the community to help with that. Thank you.

There are several UI items that break basic HIG UI rules and suggestions: for instance, tabs should be on top, “Image Cache” should be capitalized as “Image cache” (same for other similar) and shouldn’t have bold, the “frames” should be invisible, buttons should be well spaced and aligned, indentation of frame children should be the same…

A little thing not in HIG I could suggest as improvement is the memory/disk size selection. You could use a single GtkComboBoxEntry[2] with preferred values (256, 384, 512) in dropdown list. This should simplify the UI keeping simple both choosing a preset or insert a custom value.
If so, please note that following HIG the appearance should be something like

Memory size: [256_____|v] MB

and not

Memory size (MB): [256_____|v]

cheers

[1] http://library.gnome.org/devel/hig-book/stable/
[2] http://library.gnome.org/devel/gtk/2.90/GtkComboBoxEntry.html