In your case, I think they would be fine on the D drive. Ideally, the database files and log files would be split onto separate arrays with the db files on an array that is better suited for reading and the transaction logs on an array better suited for writing. In your case, you don't have many options. I have an accounting server with all of the sql db files (temp, system dbs and user dbs) on the same drive array (Raid1 I think) and the system and program files on another. It seems to work fine for them (about 10 users). They didn't want to spend a lot of money on the hardware and the transaction volume is fairly low. If the system has downtime in the evening, you can always try it one way and if needed, move the databases files to a different drive (multiple methods to do this, fastest would probably a simple detach, move the files and then reattach the database).
I have another server where I have it set up in the ideal configuration but that involves a SAN and it was quite expensive. It also supports Web applications and has an average of +2000 batch requests per second.