I'm looking for a higher res texture pack for the land and architecture of Shivering Isles. Which in your opinion is a good one to try?

You have a choice between only two that I know of.
http://tes.nexusmods.com/downloads/file.php?id=28378

http://tes.nexusmods.com/downloads/file.php?id=18677

I think you can just use the normal maps from Bomret with Vibrant ... I think. Bomrets also are higher textures, but not necessarily better. Vibrant is bright and a bit over the top (in a good way) and I think that suits SI. Bomrets more sedate.

Thanks Psymon. I'll go check them out right now.

I use both Vibrant and Bomret together - Vibrant installed first and Bomret installed second (overwriting) although you could reverse that order to your preference. As far as I recall, there may be some textures that are in one and not the other.

You forgot to say that vibrant textures are NOT highres:) and this doesn't mean that they are bad.

This is also an option:

http://tes.nexusmods.com/downloads/file.php?id=38204

Oh yeah I forgot Really Textures covers SI.

The thing with them though is that they don't always look that great with color maps and are geared for enhancing vanilla.