Website Redundancy (Hosted In Multiple Datacentres?)

Not sure where this belonged so put it in web hosting as it could cover dedicated, colo or shared.

In a couple of months I am going to be launching a new e-commerce website for the company I work for, so it is absolutely vital that the website never goes offline, otherwise I will be in deep ****.

It will be hosted on a server that has been built to be very redundant; 2x Hotswappable Redundant PSU, 4x Hotswappable HDDs (RAID 6), Redundant NIC.

Sods law though to achieve this redundancy it will need to be on a colocated server, so if there is a major problem like the motherboard dying, then I will have to get down to the DC (3 hours away) and replace the entire server. Which wouldn't be practical as we would need a replacement server, all setup and ready to go, to do that.

Also, if there is a problem with the data centre itself, then we would be stuck until it is resolved. I have hosted sites with data centres in the past that experienced power or network problems which sent the sites down and it was totally out of our control to either prevent the downtime or restore the sites.

So to increase redundancy, and minimise downtime, I was thinking of load balancing between two servers. But then I assume they would both have to be hosted in the same data centre.

I was also thinking of having the site hosted on the server, but having some shared hosting (or a cheap dedi) ready to go just to fall back on if the server goes down. That way I can forward requests to the shared hosting as a temporary measure whilst the main server is restored. But the problem with that is I would probably need to get another SSL certificate. I would need to get the payment gateway provider (protx) to change the settings for the IP of the shared hosting, which knowing them will take 72 hours to process.

Budget really isn't an issue here if it can be justified, just looking for some ideas at this stage. There is no way this site can be down for a second longer than it needs to be.