Why do some boards have both pcie3 AND pcie2 slots? Do I need both?

If your gaming will be on a single monitor, then a single X16 slot is sufficient to hold a very strong graphics card.

Only the very fastest graphics cards are impacted by pcie 2.0. And that is on the order of 2%.
Not a significant issue.

Added slots might be useful if you are planning on dual cards.
Then two running at X8/X8 will be ok.
The example you linked showed the second slot as running at X4 which would not be optimal.

Here is my canned rant on planning for dual cards:
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Dual graphics cards vs. a good single card.

a) How good do you really need to be?
A single GTX650/ti or 7770 can give you good performance at 1920 x 1200 in most games.

A single GTX660 or 7850 will give you excellent performance at 1920 x 1200 in most games.
Even 2560 x 1600 will be good with lowered detail.
A single gtx690,7990, GTX780ti or R9-290X is about as good as it gets for a single card.

Only if you are looking at triple monitor gaming, or a 4k monitor, might sli/cf will be needed.
Even that is now changing with triple monitor support on top end cards and stronger single card solutions.

b) The costs for a single card are lower.
You require a less expensive motherboard; no need for sli/cf or multiple pci-e slots.
Even a ITX motherboard will do.

Your psu costs are less.
A GTX660 needs a 430w psu, even a GTX780 only needs a 575w psu.
When you add another card to the mix, plan on adding 200w to your psu requirements.

Even the most power hungry GTX690 only needs 620w, or a 7990 needs 700w.

Case cooling becomes more of an issue with dual cards.
That means a more expensive case with more and stronger fans.
You will also look at more noise.

c) Dual gpu's do not always render their half of the display in sync, causing microstuttering. It is an annoying effect.
The benefit of higher benchmark fps can be offset, particularly with lower tier cards.
Read this: http://www.tomshardware.com/reviews/radeon-geforce-stutter-crossfire,2995.html

d) dual gpu support is dependent on the driver. Not all games can benefit from dual cards.

e) dual cards up front reduces your option to get another card for an upgrade. Not that I suggest you plan for that.
It will often be the case that replacing your current card with a newer gen card will offer a better upgrade path.
The Maxwell and amd 8000 or 9000 series are due next year.
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