Are there any texture overhauls that increase the resolution of the textures? How many are there? Could I have some recommendations, comparisons, or links please?

I advice you to download the three main ones:
Qarls Texture Replacers. Comes in many variants. I think the Redimized one is the most optimized. I think it's only available on PlanetElderScrolls.
http://tesnexus.com/downloads/file.php?id=18677
http://tesnexus.com/downloads/file.php?id=30368

Install them using BAIN so that they are easy to uninstall and reinstall and then check each one in-game to see which one looks best on your system and also check VRAM and FPS.
QTP is more demanding than the other two.

Personally I prefer Mtgr for Landscapes and QTP3 Redimized for Architecture, Forts and Aylied Ruins.
For Caves I use http://tesnexus.com/downloads/file.php?id=34175 and for Sewers I use http://tesnexus.com/downloads/file.php?id=9080

Not many, that I'm aware of. QTP, "ATP" (or, Ampol's Oblivion Makeover)... and other assorted bits and pieces.

Theres also CorePC's Vibrant collection, though not Hi-Res for the majority of the textures its a very nice set, and does have DistantLOD colourmap options for vanilla and SI in 1024 and 2048 versions. Maybe not what you are looking for but worth a mention I think.

http://www.tesnexus.com/downloads/file.php?id=18677

Scroll down the description near the bottom for the install list.

But also refer to the gamesas thread for a few updates linked here....

http://www.gamesas.com/index.php?/topic/1106036-wiprelz-vibrant-distant-lod-vibrant-textures-enhancements-vibrant-weather-wiprelz/page__p__16200138__hl__vibrant__fromsearch__1&#entry16200138

1.55 and 1.56 texture updates, snow update and the more recent Weather/sky additions

With increased anisotropic filtering and anti-aliasing (set through your graphics driver settings to force the game to use them along with HDR) this is a beautiful set.