I'm using Handcent, and have used it for years on my GNex. You can totally customize it, but you really have to drill down in the settings to get to all of the customizing settings. You can enable pop up, which is in "notification Settings--->Pop Up". You can also change incoming and outgoing font, font size, font color, background color for incoming and outgoing bubbles. You can also download free font packs in the Market, which will give you a choice of typefaces.