Because I don't have these answers, I'll speak in general terms. Because you will never really get double the performance...
of an application by doubling the amount of CPUs per system, I would rather you purchase several PCs instead of just a single system. With Intel servers, you gain much more by having several machines, each running its own applications, rather than having one super-duper PC. Each of the smaller servers could run specialized applications, which would also make it easier to administrate and tune the box for the particular application you would be using.
With one host machine, it will be more difficult to tune the box, as there may be conflicting requirements for each environment. From an availability standpoint, if that single server is down, you have nothing. If you cluster your nodes, you could be in a position of setting up a high availability environment for your critical applications. If, in the future, you notice that there are performance issues that require more resources dedicated to a certain server, you then can upgrade that particular server.
Perhaps the first thing I would do is ask your application vendor what their requirements are for running their application. I would then build my server environment based on these application requirements. Regarding specific environments, I don't think you can go wrong with either IBM or HP Servers, though my preference is IBM.
If you really want to get creative, why don't you look into the eServer i5 server platform. This server contains virtualization technologies that allow you to run multiple virtual hosts on one server (even running Linux and Windows on separate partitions). You can essentially pool your resources by having multiple environments on a single server, and pretty much automatically respond to changes in processor demand with logical partitioning, which may give you the flexible growth it sounds like you need.
This way, in a real way you don't really have to make a decision whether to purchase multiple services, as this solution provides you multiple virtual services within a single piece of hardware! Of course, you will still have the issue of what to do if the entire box goes in the night, which perhaps might even point you towards purchasing two of these goodies!
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