Is it better to use single or multiple database files?

Performance is the same when all files are on the same disk. But, you can split multi-file database on several disks, so it can speed up queries a bit if you have multiple users. However, you might already be spreading your main system, swap and temporary space to separate disks, which gives us three or four disks already. Adding more RAM in order to cache more database pages is usually much better investment than adding more disks.

Multiple files are really only needed when your database grows and reaches the filesystem limit for a single file. Otherwise it is probably not worth the effort.