Well the i7-920 is a much more powerful CPU than the A10 7800. It can also be OCed very effectively by increasing the BCLK. Most will do 3.5ghz. The D0 stepping i7-920 will often hit 4ghz. Your x58 is definitely a bit weird with 4 RAM slots. Normally the i7-920 will use triple-channels with three or six RAM sticks. It will also do dual-channels with 2 or four sticks. Since you have two slots that are not functional you might still be able to get dual-channel running using two sticks but it totally depends on which two slots are bad.
Here is a link to the motherboard's manual:
https://download1.gigabyte.com/File..._1.6_e.pdf?v=e24f694009d034c8774de469c43dd01c
On page 16 it specifies which slots to install two sticks into to enable dual-channel. There are two options: Slot DDR3_1 and slot DDR3_4 will give you dual channel. Or slot DDR3_1 and slot DDR3_2 will also give you dual channel. Pay attention to the diagram in the manual as the numbering of the slots is VERY confusing. If you can get dual-channel working with that board then you are good to go. Triple channel would be ideal, but as you say with 2 out of 4 dead slots you wouldn't be able to.
Depending on what you are doing with your system, you might not notice any difference between the i7-920 and the A10 7800. For instance, if you are gaming and you have a very weak dedicated GPU, you probably won't notice any difference. But a more powerful GPU will perform better with the i7-920.
Anyway, figure out which RAM slots are "bad". Sometimes this can be a result of bent/broken pins not making a good connection with the CPU in the CPU socket so you might investigate that. However, if you are able to get the i7-920 working with dual-channel RAM, that would be perfectly fine and will easily beat the A10 7800 in performance.