Mac Pro running OS El Capitan refuses to complete startup

Thank you; your reply has really helped, as I had no idea what was failing or happening. This computer is important to me because of a negative scanner that I need to use that works with it... At roughly the point shown in the image, the 'progress' bar stops and the loading of the software fails to complete. I cannot get any further than this. If I replace the graphics card, what would you recommend? Is it the graphics card that determines how the computer can/cannot be upgraded to the latest versions of the OS software? Thank you again for replying.

I can't get into the menu as I can't get into the computer... this is a Mac Tower; it was built to specific specifications; it has two Hard Drives (RAID array?) I guess I could get inside the case and take a look at what the graphics card is, but I'd have to get some help with that which I can do... I have thought of calling Apple and asking them for the exact specifications as the serial number of the computer is listed as one of my devices in my icloud account. Don't seem to be able to find anything more specific about the device that I can access via the cloud.

Grant; I have found a site that will supply a used refurbished graphics card that is the same as the one in my computer if I send the old one in exchange... I'm thinking however about your comment that this - ATI Radeon HD2600 XT - "is not very much of a graphics card!" Would you recommend that I try to replace it with the ATI 577? When you say it requires an auxiliary power cable - does this mean it has to be plugged in seperately to the rest of the computer? Sorry if my lack of knowledge about these things is on display here...

I have only managed to get it beyond a continous chime twice (by holding down the option key). Once I got a grey screen which flickered and was not stable, but I could make out two icons; the hard disk and another labelled "recovery...". Once I got a stable grey screen with no icons; but mostly now I can't get beyond the repeated chime. Sounds like something else apart from the graphics card might be the culprit?

The graphics card - ATI Radon that I purchased is faulty. The computer just chimed repeatedly and there was nothing on the screen. The seller agreed to refund. I put the old (Nivida) card back in; and powered on...the machine chimed and I got the stable grey screen with two icons pretty immeadiately: one labelled HD and the other recovery. I clicked the hard disk icon and got the apple logo and the bar. As usual progress stopped and I got the interference pattern on the grey screen. I guess I try again with another card? Do you have any further advice? Thank you for sticking with this!

Thank you again. I am very grateful for all of your advice. I purchased another card, fitted it, and this time BINGO! I'm back into the computer. Can't believe it - it is working as it should! The start-up is not seamless; I still get a bit of 'interference' on the screen as the bar under the Apple logo progresses, but not the same as it was, and the start-up process completes even without the alt/option key being held down. I get the El Capitan picture :) and the computer is running smoothly and much quicker than it was... tremendously relieved! Can I ask you one last question? I have been given some advice that I should keep this machine 'away' from the internet - that is not allow it to access my WiFi. Got no real problem with that as I have another computer, but I am wondering why?