My current workflow is to watch the preview of Premiere Pro and After Effects project on my PC monitor - I use dual Dell 2405FPWs powered by ATI FireGL V5000 card.

Will I benefit from viewing on the external TV over HDMI connection?

What card should I use for it?

Should I upgrade to a tri-head card (which one is recommended?), or just add a second video card with HDMI out?

I'd like to make sure that whatever I do in this regard, it is supported by Cineform's codec...

http://www.dvinfo.net/conf/cineform-software-showcase/136431-installing-prospect-w-cs3.html

Start with post #8 with regard to NVidia cards...

Adam, are you saying you got a $3,500 Quadro FX video card? What does it do that impressed you so much (beside apparently not having some issues that 7800GT may with video overlay?)?

No, I got the FX1500, about $500 at the time, only because at the time it supported overlay while the GeForces did not. With the updated driver, the point is moot, so I reverted back to the old driver.

When I upgrade to Vista 64 and more RAM, I may have to swap out to an ATI card, as I understand they still support this function with Cineform. Not thrilled about kissing off a nearly new card, but hey...

I see...

So in my case, what would you recommend?

And, is it even helpful to output to the HDMI monitor (3rd one), or should I just just continue watching the playback on the project's window stretched to take almost entire 2nd monitor,m as i do now?

I send the FX 1500 2nd output port into the HDMI on an HDTV. The images definitely have a bit different "look" than on the editor monitor. It's kind of personal preference. I like it because I think it gives me the feel of what the final TV images wll be like (all my stuff goes to DVD/BD). When doing CC, etc. I can eyeball both outputs to make my settings. It's one of those things that's helpful, but probably not critical.
Whenever Cineform gets to finishing the software, the Black Magic Intensity card will be a great low cost HDMI solution. That will take some heat off of the video card re full screen mirroring.

I do exactly the same as Robert for the same reasons. But note if you buy any NVidia card now, the latest drivers don't support this, so you'd have to go with an older driver. That of course brings its own problems and isn't for anyone who wants the latest and greatest capability.

Or you could go with an ATI card and hope they don't decide to remove a critical feature in their next driver release...