RAM Size Question... Different Pairs Together?

OK... so I am putting together an new system. I ordered 2x2Gb of RAM accidentally. I wanted to order 2x4Gb. I noticed the next day and contacted Newegg for an RMA, then ordered the 2x4GB. Both are the same, except for total size. Both sets will be here tomorrow... [See RAM Specs Below]

My Question is: Would I be better off returning the 2x2Gb [4Gb] and getting my money back?? Or, keeping all four sticks AND using all the slots at [12GB]?.. Will it work OK Together?? Does 12GB run Better Than 8GB with 2 different sizes?? [See RAM Specs Below]


I accidentally Ordered This:
G.SKILL Ripjaws Series 4GB (2 x 2GB)
240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800)
Desktop Memory Model F3-12800CL9D-4GBRL

When I Really Wanted This
G.SKILL Ripjaws Series 8GB (2 x 4GB)
240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800)
Desktop Memory Model F3-12800CL9D-8GBRL

Rest Of My New Rig:
-Motherboard = GIGABYTE GA-Z68XP-UD3 LGA 1155 SATA 6Gb/s USB 3.0 ATX
-Processor = Core i5-2500K Sandy Bridge
-Video Card = GIGABYTE GV-N560UD-1G GeForce GTX 560 Ti (Fermi) 1GB 256-bit
-Power Supply = CORSAIR Enthusiast Series TX750 V2 750W ATX12V v2.31/ EPS12V v2.92 80 PLUS BRONZE
-Case = COOLER MASTER HAF 912 RC-912-KKN1
-CPU Cooler = COOLER MASTER Hyper 212 Plus RR-B10-212P-G1 "Heatpipe Direct Contact" Long Life Sleeve 120mm
-Hard Drive = Seagate Barracuda ST500DM002 500GB 7200 RPM 16MB Cache SATA 6.0Gb/s
-peripherals = DVD Burner -Network Card - Wireless Mouse & Keyboard...

P.S. How does my parts list look??

Thanks, Wirez