your 390 onboard integrated graphics is HD graphics 2000.  It can support two monitors based on 390 tech guide. Up to 1920x1200 @ 60Hz (HDMI) Up to 2048x1536 @ 75Hz (VGA)  does your montior support DP or DVI in addition to HDMI?  it sounds like your monitor has no VGA. simple video suggested by Dell for this model: 1GB AMD RADEON™ HD6450 Dual-Link DVI Max: 2560 x 1600/32bpp @ 75Hz DispalyPort Max: 2560 x 1600/32bpp @ 75Hz VGA Max : 1920x1440/32bpp @ 75Hz Min : 640x480/8bpp @ 60Hz There are also a lot of simple video cards with two DP, HDMI+DVI, DP+DVI. If you monitor has HDMI only (no DVI/DP), to make it simple try to find a card that has two HDMI so you do not need to use adapter.  one example is sapphire R7 240. if the card uses short bracket you need to remove/modify the bracket to install in MT case. or you can get a DVI to HDMI cable to connect to a video card with HDMI/DP so you can have two HDMI.  example Dell HCVMH. Re: VGA to HDMI converter.  VGA to HDMI Converter -- DataPro
I have Dell Optiplex tower and two Sansung monitors. Both monitors work well individually using the HDMI ports. Also%2C using a spliter%2C both monitors work duplicated. I need the expanded workspace. Ports are HDMI and VGA. I also bought a HDMI to VGA connector and it does not allow detection of two monitors when I use both the VGA to one monitor and the HDMI to the other monitor. What gives%3F Please HELP. Do I need so something else%3F or just help connecting the cables correctly
Dual Monitor (help) Dell Optiplex 390