PCI-E really isn't a parallel bus - it is multi-lane differential serial bus.
You can use a single lane if you want to - 500 MB/sec per lane today, 1 GB/s
The good thing about PCI-E for this application is that you can use simple
external cables, so you can easily locate your flash units in a different
chassis than the CPU. External PCI-E connections are being used for external
disk arrays already. *Much* faster than SAS with more than one lane.
Flash is not exactly a byte addressable memory technology, btw, so you still
need to DMA to and from host memory.