There are more styles, you just need to disable the restrictions that only allow Microsoft "Digitally Signed" themes to be used.
My recomendation is to use this tool here: Home of XPero - XPize homepage
It will remove the restriction on the theming engine, add several new themes, and update all the old icons and settings left over from Windows 95, 98 and 2000.
To install new themes that you can download from here: Browsing Visual Styles on deviantART (Have to be careful here, while this is the section for Windows XP visual styles, people STILL post Vista styles here even though its got its own section).
All styles are installed here: C:\Windows\Resources\Themes
If the file you download is a zip or Rar file, unpack it into that folder.
Stay away from StyleXP and Windows Blinds. They are not needed to have custom themes and are actually a drain on your system resources, especially windows blinds.
Here are a couple of my recommendations:
RoyaleDX2 by ~avenger3871 on deviantART
Reluna 3 by ~bant on deviantART
GuiStyle VS by *yingjunjiu on deviantART
Pristine OS 1.2 by ~MohsinNaqi on deviantART
Thats it, enjoy your new interface
Also, before someone asks, Yes, you can use custom themes in Windows Vista, you just need this tool here: VistaGlazz - Reviews and free VistaGlazz downloads at Download.com
And you can download themes for it here: Browsing Visual Styles on deviantART
Thread Stickied since its pretty useful.