Uptime is nice, and we all need it. Did you know 99.9% uptime means your site won't be offline any longer than 43 minutes a month? Actually 43 minutes and 49 seconds. Pretty ease to do in my experience, even with Windows

). Now, 99.99% uptime means your site won't be offline any longer than 4 minutes a month, or actually 4 minutes and 22 seconds. Now that is something else entirely!!! But you would pay dearly for that because to provide that kind os uptime your hosts server would need to have failover setup with full clustering. Now 99.95% uptime is a good midpoint, meaning your site won't be offline longer than 21 Minutes a month, or actually 21 minutes and 56 seconds.
How do I interpret these? 99.9% uptime (3 nines) is basic. If there are problems doing this, there are bigger problems needing to be fixed. 99.99% uptime (4 nines) is acceptable for key enterprise applications, so if your website is critical, you need this at a minimum, but you'll pay for it. 99.999% (5 nines) uptime is being down no more than 26 seconds a month. Full failover, redundancy, and multiple locations are the requirements with replication every 15 minutes to 60 minutes depending on criticality. 99.9999% (six nines) uptime is being down no more than 3 seconds a month, or 32 seconds a year!! This is the penultimate of uptime, full redundancy at all levels, firewall, internet connectivity, generator power backup, redundant cooling, multiple locations, replication in real time. This is key infrastructure, mission critical, second to none service. You'll pay like Google for this.
Where do I sit for my hosting, 99.95% is all I need, for now that is. Hosts saying 99.9% uptime on big banners and sales ads - my home server does 99.9% easy so don't buy the hype.
http://www.palladiumhosting.net - 99.95% uptime guarantee - Business class hosting, best in class pricing
Also, schedules outages and maintenance windows don't count. So if your host tells you in advance (5 minutes or 5 days) it doesn't count towards the guarantee.
Wayne