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I take severe issue with "25mbit is not enough to swamp a single hard drive!".
Yes, in many cases this is true. Hell, in some cases we have customers maxing out a gige with a single SATA disk - simply because all the files they serve FIT IN MEMORY.
I have customers who max out their disk I/O with less than 1mbit of bandwidth. This is due to custom applications they run. I would say "in general" 25Mbit of "typical adult hosting traffic" will start to hurt your disk in many situations. Once you start hitting a bunch of random access on that disk, you will very quickly see a performance nosedive with very little warning in some cases. The easiest method to delay this is add RAM, as the more RAM you have the more files the OS can cache and thus not hit your disk to retrieve them.
Most sites follow the 80/20 rule. 20% of your content results in 80% of your requests. In this case, you want to at the very least have enough RAM to cache that first 20% of hot content. However has your content base grows into many gigabytes, this becomes less and less practical. This is where large RAID based systems come into play, as they spread your disk I/O across many spindles.
Basically, without seeing usage patterns no one can tell you if 25Mbit will swamp a disk or not. "In general" you'll be fine with room to spare, however large amounts of big files (movies) can quickly become more of an I/O load than you may expect.
Also remember that RAID *is not in any way shape or form* a replacement for proper backups!
G'luck!
-Phil
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