Here's my limited knowledge on the stuff -
PCI-E and PCI-X are fundamentally different busses, even though their name suggests differently. Often times, that is the first source of confusion. So, start thinking of them as separate busses (no relation to each other).
There is no 32-bit or 64-bit per say in PCI-Express. Each lane has a differential pair Rx and a differential pair Tx.
So, PCI-E x1 would be a single-lane (one diff Rx channel and one diff Tx channel) bus. Similarly x2, x4, x16 .... etc are more of the same, just in parallel.
PCI-E is source-synchronous, embedded clock, packet-based.
PCI-X, on the other hand, is a glorified traditional PCI (i.e. not packet based SerDes).
By the way, for detailed and official spec of PCI-E go to
http://www.pcisig.com/specifications/pciexpress
Even if it doesn't clear it up greatly for you, hope it at least gets u some pointers for further search.
Edited on, March 13, 2006, 8:41 PM GMT, by Sauron.
Reason: Added the PCI link