I couldn't find anything definitive, as far as documentation goes, but only the "mini-tower" version has bays for two drives, so the option may not be available in the BIOS (probably under drives/SATA operation) on other chassis versions (desktop, small form factors).  Also, you must have two like hard drives in order for the CTRL-I option to appear during POST ... CTRL-I is where you configure and manage the RAID.

Hi there,

I'm sorry to be the bearer of bad news but the Optiplex 390 comes with the Intel H61 Express chipset which has no RAID capabilities - this is why you cannot select any RAID options in the BIOS.

I hope this answers your question but if you have any other queries, please don't hesitate to get back in touch.

Well there ya have it :)  Although the Dell Optiplex 390 documentation references the CTRL-I,  it is probably a generic reference for all systems.  I almost checked earlier, but I could have sworn I'd seen it as an option to configure - must have been a different machine:

downloadmirror.intel.com/.../DH61CR_TechProdSpec02.pdf