Simplecraft (16x16) Will make minecraft run fast as hell Or If you have an epic pc: http://www.minecraftmine.org/lb-photo-realism-texture-pack/
I use LB photo realism on my Sony Vaio Laptop 256x256 But The Default Texture Pak is the best one my re made 32x32 version to make the pixels shaper.
I don't like photo realism. Anyway, I vote you try out Sphax PureBDCraft, available in all common resolutions.
If you want to keep the look of the original textures but at a higher res. check out Faithful 32x32 http://www.minecraftforum.net/topic/72747-faithful-32x32-pack-update123-support-more-123/
I strongly recommend this one: http://www.planetminecraft.com/texture_pack/16x-beta-17_01-coterie-craft-v16-regular-upd-6302011/ It's meant to be a "replacement to default" It doesn't change the "feel" of the game, just makes it ALOT smoother and easier on the eyes. example: http://aikar.co/dropbox/mc/2012-03-06_17.48.11.png
