Actually I missed the DDR2 reference-good catch! Here are the official Asus memory specs for your mobo:
2 x DIMM, Max. 4 GB, DDR3 1333(O.C.)/1066/800 Non-ECC,Un-buffered Memory Dual Channel memory architecture
4 x DIMM, max. 8GB, DDR2 1066*/800 / 667 MHz, non-ECC, un-buffered memory Dual channel memory architecture
*The chipset officially supports the memory frequency up to DDR3 1066MHz/DDR2 800. Due to the tuning by ASUS exclusive technology, this motherboard natively supports up to DDR3 1333/DDR2 1066MHz. Please refer to www.asus.com or user manual for Memory QVL
**When installing total memory of 4GB capacity or more, WindowsR 32-bit operation system may only recognize less than 3GB. Hence, a total installed memory of less than 3GB is recommended.
Here is the link to the official QVL:
http://dlcdnet.asus.com/pub/ASUS/mb/socket775/P5K-C/P5KC_QVL.zip
If a certain set of DRAM are not on the list, it doesn't mean it won't work with the board, just that it was not tested by Asus. If the DRAM meets the stated specs it should still work. If you want to run 8GB of RAM, you'll need to get 4 sticks of 2GB DDR2 1066 MHz or lower (800 or 667 MHz). Otherwise, for 4GB you can get 2 sticks of 2GB DDR3 1333 MHz or lower. If you're running 32-bit Windows, you won't be able to use more than 4GB.
I think you'll find that DDR2 is much more expensive since it's no longer in general production.