+1 to Maziar as well.
Your Celeron processor is not intended for gaming use. So it will limit your overall performance. Depending on your operating system, the 2GB of RAM may limit you somewhat as well. As deadlockedworld suggested, don't spend a lot of money on the video card, as you won't get full performance from a mid to high level card.
Also, what resolution is your monitor, and what games do you intend to play? The game you wish to play may or may not be playable with a Celeron E1400.