Poser makes zero use of SLI or crossfire so don't ever get that, even if the second card is free. You'll get no performance benefit and only increase your power bill.

As for ATI/nVIDIA, get whatever you want but stick with the 'winner' series in each case. Which means nVIDIA - 2xx, 8xxx series and ATI 5xxx series. Everything else sucks and is not worth the money you spend on it, even if the price is low.