The "938979 Vista Performance and Reliability Pack" improves the performance when copying or moving entire directories containing large amounts of data or files. Other improvements include faster boot times and fixes to Vista's Memory Manager. The "938194 Vista Compatibility and Reliability Pack" contains a variety of updates including improved compatibility with newer graphics cards, and better performance in games with advanced visuals.
Early non-benchmark tests on these patches show some interesting results, with substantial decreases in file operation times. The two update packs have been leaked and are now available for both 32-bit and 64-bit versions of Vista. Needless to say, the patch was released as private beta, so you should proceed at your own risk.