three monitors two cards?

Those two cards simply won't work together-- There is a hardware conflict when you try to run them together, which is why you got that yellow triangle in Device Manager next to the Trident.

Your best option is a new video card that can support 3 monitors. I suggested to you the ATI HD5750 in your previous post, and as you can see another gentleman agrees with me that is the best option.

And by the way, there is no such thing as an AGP 8600GT, nor are there any Core 2 motherboards with AGP. Your 8600GT and motherboard are PCIExpress 16x.

EDITED TO ADD:

DO NOT GET AN EXTERNAL USB VIDEO CARD. Those thing are very poor performers, and they are unstable. You will have just as much trouble getting it to work with your 8600GT.

you will desire to study compatibility. i detect that boards with a similar GPU often artwork jointly. Minimally a similar board producer (2 nVidias or 2 ATI's), yet even that would have issues, even if if a unusual blend works that is extra twist of fate. the terrific thank you to do it extremely is to apply a similar boards.

i think you should upgrade your graphics card. you can buy the cheapest ati radeon hd 5750 or hd 5770.If you want better performance you can choose hd 5850 or hd 5870.

you know with ati radeon new feature,it's called ati eye infinity you can use 3 monitor with one graphic cards.

That's the better solution i have.

PS sorry for bad english

I don't think it is possible on a PC but you can have more than 3 on MAC