What kind of RAM should I use for my iMac?

What kind of RAM should I use for my iMac?

My Specs are as follows:
Model Name: iMac Model Identifier: iMac10,1 Processor Name: Intel Core 2 Duo Processor Speed: 3.06 GHz Number Of Processors: 1 Total Number Of Cores: 2 L2 Cache: 3 MB Memory: 4 GB Bus Speed: 1.07 GHz Boot ROM Version: IM101.00CC.B00 SMC Version (system): 1.52f9 Serial Number (system): W8008JEF5PC Hardware UUID: EECDACBD-AF6B-5B3E-B382-2702F2AA64AD
Current RAM: Memory Slots:
ECC: Disabled
BANK 0/DIMM0:
Size: Empty Type: Empty Speed: Empty Status: Empty Manufacturer: Empty Part Number: Empty Serial Number: Empty
BANK 1/DIMM0:
Size: Empty Type: Empty Speed: Empty Status: Empty Manufacturer: Empty Part Number: Empty Serial Number: Empty
BANK 0/DIMM1:
Size: 2 GB Type: DDR3 Speed: 1067 MHz Status: OK Manufacturer: 0x80AD Part Number: 0x484D54313235533642465238432D47372020 Serial Number: 0x108343E7
BANK 1/DIMM1:
Size: 2 GB Type: DDR3 Speed: 1067 MHz Status: OK Manufacturer: 0x80AD Part Number: 0x484D54313235533642465238432D47372020 Serial Number: 0x101343E9
What sort/kind of RAM should I purchase to improve the current RAM capacity?
Should I buy an extra 2 x 2GB RAM or 2 x 4GB?
Which Slot should it be inserted to should I purchase the new RAMs?
Will it have a negative effect on the overall performance?