i think you might notice a slight speed increase, and you will probally notice a slightly higher quality to your renders due to the newer graphics card, but as for the stability, that really comes down to the software (poser) so i would doubt you would gain any stability.

if you can afford to upgrade then do, as other software you may have will definately benifit from the upgrade, vue works great with a quadcore, and i think photoshop seems smoother to me anyway.

you current setup isn't that old anyway, so i wouldn't expect to see dramatic differences if you upgrade to the setup you are looking at.