I am currently running a Gainward Phantom GTX570 for both my screens (24" 1920x1200 and 23" 1080x1920 (rotated 90deg)).

Since adding the second screen, the GTX fans have increased to handle the extra load and it has now become audible.

What is the best passive card to get that will handle 1920x1080 rotated?

It might be worth setting a custom fan profile for the GTX 570 using MSI Afterburner 2.1 from sources such as http://downloads.guru3d.com/Afterburner-2.0.0-Final-download-2562.html. This will enable you to set the fan duty cycle % to stay at minimum level at higher power draws. It will probably cause elevated GPU temps and how sustainable this is will depend on your system cooling setup.

I use Afterburner already, to overclock the card, and have used the fan profiles for previous cards.

I just fancied adding a new piece of equipment in as it has been a while...

I have been having a bit of a read and am going to try an ATI 5450 - however, not sure how nicely the two drivers will play, so it may be a bit of an experiment...

Nvidia doesn't idle the clocks if you run 2 monitors - hence the increased power used/thermal load/fan noise.

If all you want is 2D, then any modern passive card will work. The HD6450 is a nice, low power card...but I don't have any idea how well having both ATI and NVidia drivers will work.

Bought the 5450 today for £29.99 so not bad.

It works perfectly allowing the GTX570 to idle.

I was a little worried about temps as the card is so close to the Phantom (a 3 slotter), but they have been fine.

I will post some pics soon!