You know that Server 2003 is not supported on the R420, right? That does not mean it won't work, but it is possible it will not. It just means that Dell will not help you install it, and they will not provide any necessary drivers. There may be devices that were never developed with the intention of being compatible with 2003.
It would probably be best to install 2003 as a virtual machine in Hyper-V or VMWare.
If you want to try, then we need to know what RAID controller you have?