You will need to install the RAID drivers DURING the 2008R2 installation.  
I have a PowerEdge 2850 server, which when it was originally set up (by someone else) had Windows 2003 R2 on it.  However, when they set it up, the partitioned the C: drive with so little space that it's run out of space.  We've gotten a newer server from Dell and I've turned that 2850 off.  It's been off for a month or so. But I would like to repurpose it as a backup database server.  Can I install Windows 2008 R2 Server on a PowerEdge 2850?
Can I install Windows 2008 R2 Server on a PowerEdge 2850?