Everything else about your server and network is a potential bottleneck. Your disk controllers might not really support the full performance of your drives (more common than you probably want to believe). You might have a huge pile of drives connected to PCI cards (which share a single bus). You might have a switch in your network that's just not up to the task. There are really too many possibilities to list them all.
More likely, though, inherent limitations in Windows Home Server will limit you to less than the full performance of your very expensive drives. And the performance benefit of a larger cache is pretty minimal. If you look around the forums, though, you will see many people have reported that they don't have any problem streaming to multiple clients simultaneously, with some receiving music and some getting HD video.
If it's important to you to have the fastest possible hardware, in hopes that you will see some fraction of the additional performance they promise, then by all means spend the extra money. They aren't really that much more expensive.
Hard drive configuration for performance
