  Untrue. Make sure your system BIOS and ESM are up to date, and it will take up to 300GB at 10K or 15K. It becomes more important with SATA and SAS, but if you get non-certified drives, you always run the risk of unpredictable behavior, so I would recommend Dell co-branded drives if you are going to do anything important. (I might assume not just based on the age of the server you are talking about.) Just a heads up, your "other 1 for the boot drive" will need to be configured as a RAID 0 if using a lone drive.
Hello!  I recently bought a Dell Poweredge 2850 that doesn't come with any hard drives or caddies (the pictures only shows 2 place holders). I am planning on purchasing 3 caddies and 3 hard drives. The SCSI hard drives on Amazon seem to be mostly Seagate Cheetah 15K.5 300GB drives. I know the 2850 can support 300GB drives but only if they are 10K RPMs. Would these 15K RPM drives work in the server no issue? I am going to use 2 in a RAID 1 and another 1 for the boot drive.  Thanks!
Dell Poweredge 2850 and Seagate Cheetah 15K.5 300GB hard drive