The "DDR" in DDR2 stands for Double Data Rate. It means that data is transferred twice on each clock cycle. Your RAM expects a clock speed of 400 MHz and is listed as 800 MHz because the double data rate means that it transfers data 800 million times per second.
This terminology is used for all DDR RAM and does not indicate a problem. See the DDR SDRAM article on Wikipedia for more information.
Your problem running Civilization V is likely to be the graphic processor. Your PC is not designed for gaming and uses Intel GMA4500 graphics integrated on the motherboard. The minimum graphics capability for Civilization V is "VIDEO CARD: 256 MB ATI HD2600 XT or better, 256 MB nVidia 7900 GS or better, or Core i3 or better integrated graphics".
If you want to use this PC for gaming, you should look at adding a dedicated graphics card. Unfortunately your PC's only expansion slot is a PCI Express x1 slot. That won't support most modern graphics cards, but if you search on "PCIe x1 graphics" should can find some older x1 cards for sale. They won't run the latest games, but can probably handle Civilization V.