Socket AM3 uses a HyperTransport interface to the chipset, which has plenty of bandwidth. I’ve had good success with motherboards using the 790FX chipset, though that has been superseded by the 890FX now. The Phenom II X6 supports 4 GHz HT, which gives you 16 GB/sec to the chipset (assuming your motherboard/chipset also supports 4 GHz). You do see a slight drop in PCI-E bandwidth switching from AMD to Intel LGA1366 with triple channel. The memory bandwidth provided by the triple channel interface is high enough that most Core i7 systems have nearly the same performance doing host-to-device transfers from pinned and pageable host memory. (Transfers from pageable memory require an intermediate copy to a pinned buffer in the CUDA driver and generally run around half-speed.) AMD isn’t bad though. If I recall correctly, in practice I saw something like 5 GB/sec for cards in my AMD system and 6 GB/sec for the Core i7.
I think the AMD option is fine if the Core i7 LGA1366 is too much.