I have seen this before and it seems that on a 2.0 PCIE slot X8 is teh same as X16 on a PCIE 1 slot so it should be fine. I dont know why it does it but there you go, just computers being confusing again i guess.
What does GPU Z say your bandwidth is ? should be around 153.
If its reporting that correctly then i wouldnt worry.
Do youu have it in the correct slot ? some Mobo's will detect the second slot being used and just assume two cards are there and switch to X8.
Check the specs with something else as well CPU Z can get it wrong some times, Si Soft sandra Lite is quite good and is a free download that i use from time to time for a second opinion.
I looked at your manual. There is a single Crossfire switch on the motherboard; it goes one way for single card in x16 mode or the other way for Crossfire 2x8 mode.
A PCIe x8 (v2.0) slot is sufficient for an HD5870. You will NOT see a difference between x16 and x8 for an HD5870. They tested this specifically to see if the 1156 boards would be limited. They tested with an HD5870 (non-overclocked) in x16 mode then blocked part of the x16 slot so it provided only x8 bandwidth. Less than 1% difference was observed.
You can add a second HD5870 in the future if you wish with no bandwidth issues. Do NOT mess around with three cards. Two HD5870's will max out the bandwidth and likely your CPU as well for many games.
Do NOT overclock unless you will see a benefit. For example, open the Task Manager (CTRL-ALT-DEL) and set it to "Hide when minimized" and "Update speed->LOW." RUN the TM in the background during a game for at least 5 minutes. Observe the CPU level (show all threads). If no thread/core reaches 100% then overclocking is not needed. If a single thread reaches 100% it's harder to say.
Don't forget to run VSync in games. This forces the game to run at your screen refresh rate (usually 60Hz). For example, an older game I have ran at 300FPS without VSync and my CPU was at 100%. Enabling VSync ran the game at 60FPS and my CPU was at 25%. VSync also eliminates "tearing" of the screen (google it). Generating frames that your screen can't display just uses power and increases fan noise.
There are lots of tips. In general, it's better to have higher frame rates (up to the monitor refresh rate) than to have better graphics but slow frame rates and stuttering. Some effects like Shadows and AA can have a huge performance hit with minimal benefit so play around.
Yes you shouldn't get a performance hit even if its running at X4 it would be negligible, we need to get to the bottom of why your card is running at the same bandwidth as a 4870.
It really seems to me that a bios setting being wrong is the most likley cause.