crossfire vs single gpu

Solution
I5 with the single gpu. every time. You can spend less to get similar performance with a better experience in general. Almost everyone will give the same recommendation
I'd pick the second one.

In theory, the HD 7870 CF is better. E.g. 79 fps vs 54 fps here:
http://www.tweaktown.com/articles/4597/amd_radeon_hd_7870_2gb_reference_video_cards_in_crossfire/index10.html
http://www.tweaktown.com/reviews/5566/nvidia-geforce-gtx-760-2gb-video-card-review/index11.html

However, the Athlon CPU will be a problem in some games. Also, it's best to start with a good card and add a second one if needed, later - rather than starting with two lower cards.

I guess it depends on your favorite games. If they support Crossfire well, the first build may be a good idea.