Is it safe to assume a x8 card is PCIE2, and a x4 is PCIE3?
Page after page just lists the slot length. 
Link to card? Model numbers?
No, really just a general question, ty.
No, most of the pcie3 cards I see are x8 if they have two modules.
Single module cards can be x4, but sometimes are x8 simply because it’s (usually) one of a few types of knockoffs mass produced with one board.
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I wouldn’t worry about it too much, 4 pcie2 would be 4x500MB, or 2GB/s, which means the card would need two SFP+ 10gig sockets, and struggle really hard to flood the 2GB/s (16Gb/s roughly)
I saw a lot of cards that had PCIe gen2x8, and to be honest they were like 40-70% cheaper than the Gen3 ones.
I would look more at compatibility with the system you want to use it on.
Like @Log mentions, it seems a lot of the cards are based on a few base models, just rebranded with specific end vendors logo.
FreeNas for example might be really compatible with LSI 2008 chips, but might refuse to co-operate with high point card.
I presume new versions of MS server will have the drivers to just work with cards new enough to have gen2 or newer, but should probably check your desktop OS for compatibility
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