Yep all quad core CPUs work in 32 bit and 64 bit both. In fact all such CPUs are 32 bit by default and only switch to 64 bit mode if the OS asks them to. So you wont have any issues in Vista 32. You really should consider going for Vista 64 though so you can make use of all your 4GB RAM.

As for poser improvements, if you have Poser 7+ then you will see about 1/2 - 1/3 the render time from a single core. In some cases it can go even better but depends on the scene. Other versions of Poser, 6 and below, will see very little improvement.