1. Workstation graphics cards for Poser are a complete waste of money. Those cards come with specialized libraries for high end 3D applications (3DS Max, Maya), and Poser does not use those libraries. The only advantage that they offer is better stability: their OpenGL drivers are better and the hardware has been tested before the card went into the shop. But the performance of a $4000 workstation card is about the same as that of a $200 consumer card.

2. Large video card memory is somewhat useful in texture shaded preview mode. On the other hand, reducing the maximum preview texture resolution also works. 256 MB on a graphics card is low end these days, 512 is mainstream, but 1 GB usually isn't worth the extra dollars - unless you're an avid 3D gamer.

3. You are completely right. You do want a reasonable 3D performer for Poser preview, and depending on your Photoshop version Photoshop can also make use of hardware acceleration on the graphics card. I'd say a GX260 is overkill. An nVidia 9600GT or 9800GT will do the job much, much better than your current ATI, and they're far more affordable than the GX260.
   The ATI high end cards (4850/4870) have a better price/performance ratio than the nVidia GX series, but they run far too hot to my taste (55 celsius idle, up to 95 celsius under load). The cards themselves have been designed to run at these high temperatures, but that heat will also warm up the southbridge chip on your mainboard - and if you have an nVidia based mainboard, that southbridge produces plenty of heat all by itself already. Putting a hothead of a graphics card close to that southbridge is a recipe for disaster - expect your southbridge to burn out within weeks.

4. The graphics card has no influence at all on render speeds in Poser. Period.
   There is a non-realtime plugin render engine for 3DS Max, Maya, and a couple of other high end applications that does use the graphics card, however. It's called RT2, and the render speeds it achieves are unbelievable: 50-100 times as fast as Mental Ray, even on a modest graphics card. Alas, it's around $500 for the render engine alone, and it's not available for Poser.

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