are u using the startup disk (in my country it is white in color) to do the installation
Do you have the correct drivers for the hard drives for Winodws Server 2008? You may have to slip them into the installation media.
You need to use startup CD in which the PERC controller drivers are available and that CD uses it to configure the BIOS setting on PE 2950 also it sets your RAID level of hard drive and does initial setup. You need to install Windows 2003 x64 first and then try to upgrade it to Windows 2008 which works and I tried doing the same on Del PE 2950
All the best,