What OS will you be running?
With a 60GB hard drive, I wouldn't worry about splitting into two partitions myself. I would just leave it as is. Since you do want two partitions, I would do at least 20GB for the OS (C) & the rest for the D Drive. 20GB will have enough room to store pretty much any OS. You could take the easy route to and just do 50/50 for two 30GB drives.