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It depends on the RAM -- 2x1GB and 2x2GB; if it's the same Type, CAS, Frequency, and Voltage then it can work and Dual Channel 'can' be supported; 3GB per Channel or 6GB total. Also, you'll need a 64-bit OS to access and use >4GB of RAM. In general I caution mix-matching RAM and it's best to simply purchase compatible matched sets. RAM particularly DDR3 is very cheap today and your data integrity IMO more valuable.
I'd need to know the EXACT RAM of both Sets; please provide link(s) to each set.
Q - RAM links?
Q - MOBO?
Q - CPU?
You can use the free CPU-z to identify; CPU-z -> http://www.cpuid.com/softwares/cpu-z.html