What should I get?

[Magnetic]

This:
NCIX.com - Buy Kingston HyperX PC2-6400 2GB 2X1GB DDR2 800MHZ CL4-4-4-12 240PIN Dual Channel Memory Kit - KHX6400D2LLK2/2G In Canada.

That:
NCIX.com - Buy Kingston HyperX PC2-6400 4GB 2X2GB DDR2-800 CL4-4-4-12 240PIN Dual Channel Memory Kit - KHX6400D2LLK2/4G In Canada.

Here is my dilemma, I'm using a gigabyte ga-ma790fx-ds5 and I already have 2 sticks of the first ram. If I buy some more I will save some money however I'm not sure my system will be stable as I will have filled all ram slot and the board has one phase deditacted to memory. I'm not sure of the quality of the mosfets used so it might not be an issue(more does not necessarily mean better). The ram also needs 2v to work at those timings.

Other option would be replacing the ram I have with the 2nd model spending more but being sure it will work.