If you are talking windows maybe. But we are talking Linux, and it is my understanding that 32bit Linux doesn't have the 4gig limit. I suspect that they use some type of memory paging scheme to get around the limit. Also, mining programs are not mining intensive so even is your do lose RAM space to GPUs it is still viable. If you are USB mining, your are only limited by the USB controller's ability to handle a certain number of devices. I've read here that adding PCI USB cards can get around that problem since they have their own controllers.
Regardless, 32 vs 64 doesn't seem to be an issue. I suspect that, with the right software a 16 bit machine could be a viable miner if it wasn't a power hog.
http://en.wikipedia.org/wiki/Physical_Address_Extension#Linux
But.. srsly it looks like a ugly hack. Stick with 64-bit if your cpu supports it. Better support also.
Each USB bus can have 127 devices. AFAIK doesnt vary by controllers ability. You just have to provide enough power to it... Some controllers have 4 ports on a single bus, meaning they internally have a 4-port hub attached, so now per port can support fewer devices, but the total for the bus is still 127. The PCI USB cards have their own bus(es).
