Instead of the simple beep signifying a hit, perhaps a rising (or falling!) tone to indicate how much (overall) damage the target has suffered, so that as it takes more damage, the tone changes?

Could work, but since the hull meter is color coded, it's not that hard to glance up between shots.

I think visual cues are better for status things like target health. Maybe a health meter could be superimposed on the target box somewhere..

yes, unless you have perfect/relative pitch, most people probably wouldn't notice the difference.
"target has full health *the note A plays in the background, 440hz"
"target has suffered damage *note drops to G"
hehehe.... yea... if it is a long duel I wouldn't be able to tell, relatively, how much the pitch has dropped over time.

I kinda like the idea of a small damage meter above or below the target

I don't think you need another damage meeter on the taget box. The HUD one is enough. I do think that the sound that plays when the enemy takes damage is distracting. It sounds like you are taking a hit instead. Have Michael Dorn Saying "Direct Hit Captain" that would be cool, but unrealistic. Maybe a less jarring sound, one that sounds more of a "winning" tone, less alarming.

one idea for the HUD. have a picture of the enemy ship style, similar to the one of your own ship. just so one could see what the ships look like when they are far away.

where I work (in an operating theatre) the pulse oximetry, as well as giving a visual display (as a %age) also uses the audio cues, so you can tell which direction you're going without actually knowing the figures...just thought it'd help in here too...

Sure, that's an operating theater, where surgeons need all the cues they can get. And a pulse is (hopefully) something that is constantly going and needs constant monitoring. Like harvestmouse said, unless you have constant reference (or *absolute* pitch, perfect pitch is different[1]), a tone pitch cannot accurately show a status.

But it's not a bad idea. I wouldn't find it that useful, but maybe others would.

[1] http://www.stanford.edu/~meep/abspitch.html

On my 21" screen, I find it very annoying that I have to glance up-right quite far just to see the target status, or my status. I would like to have those closer to the centre, where I want to focus my attention keeping track of the enemy position, the target lead and my own heading.

a thin bar above the hud for the enemy status and one below the hud for my own would rule!

On my 15" screen, I refuse to give up any more screen real estate for extra bars.