External drives will mean slower loading and saving. As for rendering I'm not sure it will matter all that much.

Drive speed matters during rendering when paging is necessary so the faster your internal system drive the better performance you will get when rendering big scenes. The location of the file (internal or external drive does not affect this process)

Systems with over 2gb of ram are of no real benefit to Poser as it can't use more then 2Gb anyway.
If your renders consistently require more than 2gb of ram you may want to invest in two high speed SATA drives and put them on a stripe RAID. (You will double the speed of caching and retrieving from the drive)

I hope this helps.
Mano