That's a pretty decent setup. It should run Poser 6 with ease. The graphics card should do fine, the CPU is fast enough, and you have enough RAM. A little bit more never hurts, but 1.5 GB should be enough to run P6 comfortably.

First, do you have Symantec or McAfee virus scan running in the background? If so, 30% of your computing power is gone. I NEVER run a resident virus scanner, I always do manual scans, for this reason.
Other virus scanners are less greedy when it comes to system resources, but they all take up RAM and CPU cycles.

Second, you can speed things up a lot by changing the graphics card settings. In the nVidia control panel, you can set the performance/quality settings per application. For Poser it's best to set all those setting to maximum performance. No 8x antialiasing, no trilinear filtering, etc. etc.

Third, in XP you can also set some display properties. Right-click the My Computer icon, choose Properties, tab Advanced,  and the Performance button. Set the visual effects to "Adjust for best performance."

In this same dialog you can adjust your page file/virtual memory settings. It might be that you have a heavily fragmented page file - the Windows defragmentation tool can't handle that. Set the page file manually, maximum size and minimum size equal, at 2 times your physical RAM (3072 MB in your case). 
Best to do this right after a defragmentation run. 

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

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