Alright due to your specs, you may not be able to record videos very well. How much RAM does your computer have, and what GPU are you using?
A duo-core processor tends to lack the power to run a game and record the game at the same time. I have personal experience with this issue, dual core CPU's can't process as many things and as fast as a multi-core CPU can.
I would recommend Fraps, as I personally have used it without problems.
With a duo-core processor the best you are gonna get is maybe something around 15-25fps record on the lowest settings your game can go. Or in some cases, like recording League of Legends for example, I can record probably around a good 20-40fps with fraps. I have a duo-core myself although its 2.70 GHz.
Fraps is simple and records well, I recommend using the full version. Although, small file sizes may be hard to attain.