I could not find a single benchmark or review of the 770 chipset boards, much less the specific one produced by MSI, or any desired Crossfire benchmarks.
However, I can't imagine Crossfire ever being crippled to the point of no improvement noticed as a result of the chipset's x16/x4 limitation...; if you already own 2 cards, and your PSU is sufficient, a few benchmarks of any FPS game from the last 1-3 years should tell the story pretty quickly.
You will get more performance from 2 4670s in any speed compared to 1 4670(except in games that don't support CF)
However,x16x4 is the slowest setup for 2 cards and it performs significantly lower than dual x8/x16.
The most cost-effective upgrade for your PC is to upgrade to 1 single powerful card.
Actually not all games support CF. As a CF user,I've to say there are many games that run better with CF turned off like PES 2011,SC2, BioShock 2 also there are some games which give texture glitches with CF enabled and you have to tweak the settings and install crossfire profiles(or use an app called Radeon pro to tune things up) in order to play the game without any problems
OC'ng your card will give you a performance boost but it's not anything significant
@Maziar, i know that many games do not support CF, but most of the games that will come out in the future will support SLI and CF. Also the games you listed are easily run with 1 card (not sure about BioShock 2, but BioShock (1) wasn't very demanding)