Are your disks on a SAN or physically attached to the machine?
SAN disks in a good SAN should be set up in RAID by the SAN controller and should be transparent to you. The other fun part with SANs is that you generally don't need to ask for separate drives as the SAN controller won't usually give you 1 drive when you ask for a chunk of disk. And even if it does, it could change that based on disk use.
If you have physically attached disk, you want to try to keep all of the high I/O operations on a single disk where possible. What I mean is 1 disk per high I/O database.
As for the type of disk and RAID, that depends again on your disk I/O, capacity requirements and budget. If you find your disk I/O currently is not a bottleneck, then increasing the disk I/O (such as from moving from 10,000 RPM HDD to SSD) may not be the most bang for your buck. And RAID-ing your disks for performance may not be that beneficial. On the other hand, if your bottleneck is disk I/O, then moving up to SSD and RAID-ing for performance may help.
Without knowing a bit more about where your bottlenecks are, it is hard to know what would be a "good setup". It may be partitioning your data across multiple disks would be the best bet, or it may be better to have everything on one disk.
Depending on how much swap space you need, you may want a second disk for that. But be cautious about the C drive swap space; some things will go wonky on you if you disable swap space on the C drive...
My only concrete recommendation would be to put your backups on a different computer altogether. Don't want that PC to have some weird bug that causes all drives to die on you. Had that happen on a personal PC... power supply was too small and eventually caused 1 drive to spark on the case which fried all drives on the controller. At work, we had a 3rd party tool handling our filesystem (wasn't NTFS, it allowed for a single disk to share read/write access across multiple physical servers. It was used for failover) and the tool did a bad write on one of the 3 servers and corrupted the entire disk.
My recommendation is to have the backups on a different PC (offsite if possible) so they are more secure. If your PC holding the backups explodes, you have no backups.