I've got three 24" Asus 2ms response time screens waiting for a Mac Pro to power them with either 3 GT120 cards or one ATI 5770 card.
With those three monitors...a Mac Pro would definitely be better!
The GT120 provides everything I need, and is apparently more reliable in a 2 or 3 card setup. I can get 3 monitors running on only 2 GT120 cards, but 3 cards just for the heck of it
When you mentioned having 3 GT120 cards (and three monitors)...I was going to mention that you only needed 2 GT120's (for three monitors)...since each GT120 can drive two monitors...but then you mentioned knowing that.
For what it's worth...I mentioned a couple of times in the past that I had a 2009 Mac Pro (that I purchased probably 2.5- 3 years ago)...and sold it probably at least 18 months ago.
I had 2 GT120's in it...and just for fun (one time) I hooked up 4 monitors. It was kind of neat for about 15 minutes...but it was a bit overwhelming...and 4 monitors was much more than I needed.
As far as pricing. For my 2009 Mac Pro (3 years ago)...I paid $1150 for it. Which was a
terrific terrific price at the time. I think that Mac2Sell had it's estimated value (then) at around $2200. The seller was asking $1600...and I guess that they were motivated to sell it...and I got it for $1150. The seller was a college professor...so I'm assuming that they had at least a basic idea of its value.
I really wasn't using the computer to it's full potential...and I really wanted a newer 17" MBP at the time (my main computer now)...so I sold it to get the funds together for the 17" MBP. I think that I sold it for $1500-$1600 (I forget the exact price now). So I made a few hundred on the sale...but still sold it for well below what it was worth then.
I haven't done any research lately...but I'm thinking that $1150 for a 2009 Mac Pro may still be a decent price even now...2.5+ years later after I purchased it.
About 3-6 months ago (before the new Mac Pro was announced)...I was doing some casual price checking on e-Bay on first gen. Mac Pro's (which I know that you're not interested in)...just to see how the prices were fluctuating. Since dual-cpu Powermac G5's were selling for around $100-$150...I thought maybe the price of 1st gen. Mac Pro's were dropping as well.
To the best of my knowledge...the price of 1st gen. Mac Pro's (about 9-12 months ago) seemed to bottom out at around $550 bucks. But when I checked about 4-6 months ago on e-Bay...the price of 1st gen. Mac Pro's seemed to be increasing somewhat. So I figure that if the value (at the time) of 1st gen. Mac Pro's was increasing...that would also push the price up on Mac Pro 2,1's, 3,1's, 4,1's...etc.
So I'm not 100% sure where prices are now for used Mac Pro's...and how the announcement of the new Mac Pro coming out has affected things (up or down).
I'm assuming you've done some research already. If you were to get a Mac Pro 4,1 (the oldest Mac Pro that meets your needs)...what are you expecting to pay (range)?
Assuming a modest stock setup with a GT120 video card, a single HD, and 6gig of ram...and average cosmetic condition (no serious dents, ding's, or bent handles).
- Nick