Poser 8 works fine with Windows 7 64 bit (pre release version) on the i7. It is faster as the quad, but more important, it has 8 cores and you can limit Poser 8 to 7 cores  for rendering and still use the machine without delays for other tasks. Poser 8 itself is still 32bit, but in the 64bit Windows 7 (and vista and xp64) you have 4GB available within the application, which goes up from 2GB in the 32bit versions.
In the next PoserPro there will probably.a 64bit render engine which allows larger renders.

If you can afford it, go for the i7 with 64bit Windows 7