This would be an excellent computer for surfing the internet, working on Word and Excel documents, playing music and other moderate tasks. Not to bad for under $200 (a guess based on the parts you've listed). HD video might play slowly on this system, based solely on the onboard graphics that comes with the G31 chipset (Intel GMA3100 is kinda low end).
For around the same money, you'd be better off with the FOXCONN M7VMX-K. The Nvidia onboard graphics are better and it has most of the same features as the G31. Using the same other components, it should be good at all the same things, but play HD video better.
As for gaming, this rig wouldn't quite have what it takes for COD 4, let alone 5. As most video games are not overly CPU heavy, the E1200 should be okay for gaming, but neither choice of onboard graphics would do much more than give you a slightly amusing slideshow. You would need a dedicated graphics card, something in the $125 and up range (preferably way up, like around $200+ range). Nvidia 9600GT is a standout graphhics card at around $125 and should be okay for most games, certainly COD 4.
From my research, to stay under $200 and get decent gaming that would outperform the Intel system you're proposing, you may need to go AMD. Picking up a AMD 780G motherboard, an Athlon 64 X2 5000+ CPU and a Radeon HD 3450 graphics card is still under $200 (with RAM added in) would run most moderate tasks equally as well as the E1200 G31 combo, but have the benefit of Hybrid-Crossfire for gaming.
With the AMD 780G chipset, there is support for all the latest AMD and Nvidia graphics cards and the latest AMD Phenom processors for later upgrades. I'm an Intel fan, but the 780G is making me reconsider.