Poser 7? I'd say XP 64 bit - if you can get the drivers for your audio, graphics card, network card. 
I put my desktop systems together myself with XP64 in mind (run XP64 on all of them), which means

• nVidia or Intel chipset (not ATI or VIA)
• nVidia graphics card (not ATI)
  a GOOD brand of mainboard (MSI, not ASUS or - shudder - Asrock)

Everything works fine, and Poser 7 can use up to 4 GB of address space on XP 64 bit.

You can also get Poser 7 to use more than 2 GB of address space on XP 32 bit (Pro edition, doesn't work on Home). Just add the /3G switch to boot.ini and XP Pro will allow up to 3 GB of address space per process.

Poser 6 and older will not use more than 2 GB of address space, either with or without the /3G switch in boot.ini. For Poser 6 or older switching to a 64 bit OS is no use at all.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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