As a shopper, is there anything in particular you should look after when it comes to a good video card for photo edit/preview? I would assume a DisplayPort might be beneficial, but are there some chipsets that are viewed as "better" when it comes to dynamic range or is it up to the monitor?
It actually really depends on the software. It is especially important to do some research here because some graphics card have really big power requirements, and your computer might not be able to accommodate any of them. Make sure to compare the specs of your power supply to those of the graphics card.
To my knowledge Lightroom is not GPU accelerated (see requirements), but it is heavily multi-threaded (the more cores, the better). Apple Aperture is accelerated in that it uses "Core Image" on Apple hardware (see requirements). As Matt pointed out, some parts of CS4 are GPU accelerated, and this is even more true for the whole CS5 suite. If you have a Video DSLR like the Canon 5D MII, processing large photos and huge videos can be very tricky without the proper hardware. With the introduction of the Mercury Playback Engine in CS5 Adobe delivered some serious hardware acceleration; if this is something you see in your future, you might want to check the list of supported cards for CS5 early on (and how to enable more).
As a rule of thumb in Photoshop, the more memory on the graphics card, the more "accelerated" documents you can open at the same time. Opening one large photo (say > 20MP) shouldn't be any problem with any modern graphics cards.
I haven't heard of any differences between LUT performance between graphics cards either; as long as your monitor is properly calibrated, you should be fine.
The graphics card can be very important when it comes to editing photos. Most current photo editing programs e.g. Photoshop CS4/5 use the graphics card to do some of the processing, taking the load off the processor.
You don't need a monster gaming card, most low to mid range consumer cards will give you an advantage. If you plan to use Photoshop it's worth checking the card has been tested by Adobe as I saw major glitches with my card when CS4 was released, see:
When it comes to the quality of the output if you're using a digital connection (e.g. DVI) then the graphics card shouldn't make a difference as long as the computer/monitor is calibrated. Most calibration systems use a look-up-table (LUT) on the graphics card to translate the colours from Windows to a signal that will produce the correct tone on the monitor. However I have never heard of any differences between LUT performance between graphics cards. Some new high end monitors geared toward graphics editing have their own inbuilt LUTs for colour correction so you can perform calibration on the monitor hardware alone, although performance can be sub-optimal due to not calibrating the back light correctly compared to software (+graphics card LUT) calibration, see
As you suggest it's really up to the monitor as dynamic range and contrast vary greatly amongst monitors and between calibrated/uncalibrated setups.