Hello we have a R510 with following config

Intel Xeon E5620 2.4Ghz, 12M Cache,Turbo, HT, 1066MHz Max Mem

Intel Xeon E5620 2.4Ghz, 12M Cache,Turbo, HT, 1066MHz Max Mem

24GB Memory (6x4GB), 1333MHz Dual Ranked RDIMMs for 2 Processors, Optimized 

Seems like there's 2 extra slots that's can be used, I'm wondering if we can just purchase 2 x 16GB
http://accessories.us.dell.com/sna/productdetail.aspx?c=us&l=en&s=bsd&cs=04&sku=A6996789&mfgpid=204325

Or do we need to replace all memory?

What would be the most economic way to add more memory?