The two different sizes don't necessarily cause conflicts but it can run slower as a result as the motherboard will run a slower timing (lowest common denominator). Typically there are less headaches running 2 DIMMs instead of 4 mismatched ones and it is easier to optimize the timings to make them run faster.
I would just get rid of what you have (sell it, donate it etc) and 2x4GB DIMMs as your upgrade. Unfortunately your i7 board will probably by DDR3 so you'll have to get rid of all the memory and start over.
Or ... try to limp along on the 4GB you have until you can do the full upgrade. I have 4GB in this computer running WIN7 64-bit and don't have too many issues as long as I'm not running several heavy duty apps at once. At work I have 8GB and get out of memory warnings all the time, LOL. I also upgraded to an SSD OS drive at home and that made a big improvement (plus that upgrade would carry over).
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