PCI-E Slot problem

Hello
I recently upgraded my hardware and... hmm everything seems to work fine BUT i think my graphics card could work better.

ok, first things first - specs:
mobo: Asus M3N-HT Deluxe(mempipe edition to be precise)
cpu: AMD 955 3.2GHz
hdd: WDC750GB 32MB Cache and WDC500GB 16MB Cache
gpu: BFG GTX 285 OCX
psu: Chieftec 750W
sound card: SB X-fi Xtreme Gamer
os: Windows 7 Ultimate x64

Now the problem:
Well my mobo has 3 pcie x16 slots (two blue and one black between them) The problem is I can install my new gpu only on black slot(which can only run at x8 from what I found out, also gpu-z shows that) If I put this card in either top blue or bottom blue slot and when i power it up everything seems to work except theres no screen but I can hear the bios signal and I can see that gpu fan is working....
I searched the mobo manual for any clues but only found out that the primary pcie slot is top blue(maybe it can be changed in bios? however I didn't found anything like this, by name at least)
Can someone tell me what is going on and how to fix it ?

BTW: I wonder if this is worth my time anyway ? Should I expect any changes in performance?

well you can read it here: http://www.asus.com/product.aspx?P_ID=RgHWsXGQyHAG1n7Z
and where did u read that i can install my gpu only on black slot ?