Minecraft 512x Super Hi-Res HD Texture Packs (512 x 512)
For quite some time, the accepted normal pixel cap for Minecraft texture packs was 256 x 256. But boundaries love to be pushed and now there are several 512 x 512 texture packs floating about the place inspiring envy and locking up people's computers.
The thing is, of course, that 512 resolution graphics have the capacity to look fairly grainy and almost unrealistic in spite of their alleged realism. If I've learned anything about Minecraft and Minecrafters however, it's that there is great diversity of opinion when it comes to aesthetics. Some people refuse to play with anything besides the default 16x textures. They'd sooner mine their own face than install another texture pack, even a 16x one. Then there are people who chase ever higher resolutions, and like Icarus flying too close to the sun, they are sometimes burned and go plummeting to earth in a ball of wax and feathers.
So what's actually available in 512x? At the time of writing, not a whole lot. People have been having a hard enough time getting 256x packs running, but that doesn't mean that there won't soon be a whole host of super hi-res texture packs out there. One you can start with is LB Photorealism, it's already a wildly popular texture pack and it's now even more popular that you can push the boundaries of what can be done in Minecraft.
Aageon also has a 512x work in progress, he previously made Drasavil, which is one of my favorite texture packs because of its tendency to high art. You don't get daisies, you get lilies and crocuses, that's what you get.
There's also a lesser known 512x texture pack called 'Vector' and it is remarkable in the fact that it doesn't strive for realism, it strives for high quality instead. However even the creator admits that the 512x version will crash most computers, which leads me inexorably to my next, important point.
Note: If you don't have a decent amount of RAM, you can forget about running a 512 texture pack. 2 GB won't cut it and even 4 could be pushing it. Minecraft is, and always has been a memory hog. If you do have the RAM to handle higher resolutions and you're still running into trouble, there are ways to increase the amount of memory allocated to Java however and they can be found here at this article on how to allocate more memory to java in windows.