I'm going to be running a quad-boot setup on my pc. Say after partitioning all my OSes and after I have booted up in whatever OS, will I be able to access data from other partitions on that drive and/or partitions on other drives?
As long as the OS supports the file system used you will be able to read, and possibly write, data on other partitions on any disk. You obviously have to be a little careful that you don't accidently destroy important system information whilst doing this. An OS normally protects it's own important files, but won't know what files are important on the partition containing another OS.
If any of your partitions contain Windows, install that OS first, in the order Vista->XP->2000 to ease problems setting up the bootloader.