Yes, P5 should run on your system, but I wouldn't recommend it. The performance will be severely lacking, and you won't be able to get much use out of the Cloth or Hair Rooms.

Frankly, I'd suggest a hardware upgrade even if you settle for Poser 4/Artist. At the very least, install extra memory. If your motherboard will support it, an additional 256MB shouldn't cost more than $40-45.