I'd like to follow up on the pretty good answer Katherine Villyard already put up.
It somewhat depends on the intended usage of your database.
If you're expecting a lot of write operations, go ahead and put your
.ndf files on the faster disk.
If however your database is either one that is generally quite static (serving web content for example). And the queries don't vary much, chances are you'll get a large amount of the queries in your memory, or even cached on the application side. At which point you are better off using the faster disk for your
tempdb and backups.
Likewise, if you're expecting a lot of large queries, such as for an
OLAP database, you're better off storing your
tempdb on the faster disk. And putting the
.ldf on your slower disks as it won't often be part of the bottleneck.
In any case, don't bother with putting the binaries on the fast disk, we generally put those on a slow (not the system if it can be avoided) disk.
Also, don't get hung up on trying to get both the
.mdf files on the fast disk, generally they're separated whenever possible.
So in summary, review your load to see what will be your most likely bottleneck.