100% uptime is entirely possible but all depends on the way the hardware is setup, what redundancy is configured and so on. If you are talking a single dedicated server though it's going to be hard to offer a solid guarantee. If using raid 10 then drive failures are less of a worry however what if the CPU or Ram pop? It then comes down to how fast it can be replaced and brought back up. Might be 10 mins but could be 6 hours which heavily impacts on the uptime.