Where can one find quickly what is the maximum resolution that a graphics card supports?

For Nvidia based cards of all varieties I would have gone straight to the Nvidia website rather than the reseller (EVGA, XFX or whoever) and checked out the specifications of the card there.

Picking one at random the Geforce GTX 570 specifications page immediately lists the maximum resolutions for both VGA cable and digital links:

2560x1600 Maximum Digital Resolution
2048x1536 Maximum VGA Resolution

And it appears to be true for pretty much all of their graphics processors. Just find their specifications page and the information is right there.

Similarly AMD do the same for their graphics processors, so if you are looking at a Radeon based graphics card then again just completely ignore the reseller and go straight to AMD and find their specifications sheet.

Picking one at random again the Radeon HD 6970 has, hidden near the bottom of their specifications page (in amongst a huge wodge of information you'll not care about) shows:

DisplayPort 1.2
    Max resolution: 2560x1600 per display
    Multi-Stream Transport
    21.6 Gbps bandwidth
    High bit-rate audio
HDMI 1.4a with Stereoscopic 3D Frame Packing Format, Deep Color, xvYCC wide gamut support, and high bit-rate audio
    Max resolution: 1920x1200
Dual-link DVI with HDCP
    Max resolution: 2560x1600
VGA
    Max resolution: 2048x1536

So basically just ignore the reseller, as they'll just be throwing impressive sounding numbers at you in a bid to get you to buy their products, while the actual chip makers will tell you what it can actually do.

Basically just search either the Nvidia or AMD sites for whatever graphics processor you are looking at and you should, somewhere on the specifications page, find the information you are after.

-=EDIT=-

For Intel chips it seems there is a bit more work involved. Searching for your example (Intel 910 graphics) took me to this page and looking theough the whitepaper pdf at the bottom it has a section titled Intel ® Graphics Media Accelerator 900 Architecture where it lists the device capabilities. It's nowhere near as easy to find as Nvidia or AMD cards, but it is there

In addition to supporting CRTs via a VGA connector with a maximum pixel clock of 400 MHz (up to 2048x1536 resolution @ 85Hz refresh rate)

I can't seem to find anything quickly regarding Sandy Bridge though...

In all honesty I would be surprised to find any current graphics solution that doesn't support up to 2560 x 1600, only older chips would likely have problems running that high.

In Linux use xrandr to query the card:

tim@MushaV3 ~ $ xrandr -d :0 -q
Screen 0: minimum 320 x 200, current 3840 x 1080, maximum 16384 x 16384

As you can see, my card can go upto 16384x16384 but the colour depth will suffer

I originally posted the above while working on another question and coming accross this one. I now realise that while the above is 'correct' under certain circumstances the only generic way to be sure is either via trial and error (keep upping the res until you find one that doesn't work, then drop back) or finding a spec sheet which lists the exact supported specs.

I came to this realisation after for the other question I found that the supported resolutions and refresh rates for 'external' devices were entirely different to that shown to be supported for the 'internal' (i.e. laptop) connected monitor. Simply because a card supports a resolution over one one interface doesn't mean it will do the same over another.