Help on building a new Server
If this is local storage, layout matters a lot more. If it's SAN storage, things are a little bit different.

Here's what I do for local storage:


Put big data files (mdf, ndf) for user databases on non-SSD storage
Put log files (ldf), and tempdb (all files) on SSD


Data files are typically around 80-90% reads to writes. You can mitigate the "slow" part of reading from them by, as you say, "spend[ing] a lot on RAM". RAID 5 or 6 is the usual choice here.

Log files and tempdb are different, and they need to absorb a lot of writes quickly. They're a much better fit for faster tiers of storage. RAID 10 is the usual choice here.

The system drive storage class isn't terribly important here, but I'd make a note that backups either shouldn't be done locally, or they should be copied to redundant storage. If you lose your backup drive, you, well... Don't have backups.

If this is SAN storage, what matters even more than drive layout and storage tiers is making sure you have adequate networking paths between the server and the SAN.

For more details why, see this Q&amp;A: SQL Server has encountered occurences of I/O requests taking longer than 15 seconds.