Use Vista 64 bit on both ends and all issues are resolved.
I've been trying to figure this out for a long time now, when you have 2 pcs with shares on a 100mbit network, you can fill the network connection to 100%. When you're using a gigabit conneciton, I had not found a way to actually use most of your connection. I've used 2 pcs with RAID arrays on each end, and yet you can only get about 25% of the connection used at best with xp to XP or Vista to xp. Turning on jumbo frames the best I've seen is about 35%. It's not a CPU issue, it's not a drive limitation, it's not a network limitation, so the best I can conclude is that it is either something 32 bit or some software limitation. It might also be hardware, but I can't confirm or deny that yet. If you have two drives on each pc and do 1 copy from drive a to drive a and one copy from drive b to drive b at the same time I've seen it hit 50% using XP 32bit clients. Whatever the issue is Vista 64 is no longer bottlenecked.
The next test I'll have to do is Vista 32 to Vista 64 and see if that still rolls along.