About UrbanCraft Resource Pack
This texturepack WILL MAKE YOUR NORMAL WORLDS LOOK WEIRD. Very much so actually. But it will make the UCP buildings look awesome. Of course you can use this in creative mode to build epic modern stuff but always keep in mind that these buildings will not be compatible with other texture packs.
In order to use HD texture packs properly (32× and higher) you will need to either patch your Minecraft using MCPatcher or using OptiFine. Optifine is preferred as it includes all of the features of MCPatcher, and greatly increases game speeds and graphics settings. MCPatcher tends to be updated on the same day as Minecraft updates, so use it instead if you are in a hurry.