You're not planning on using anything from the old computer?
All other things being equal, any dual-core will be faster than any single-core, no matter what the speed. In other words, 1 dual-core at 2GHz is faster than 1 single-core at 4GHz. There are many other factors, like memory speed, hard drive speed, but most important for gaming is the graphics card. The one you have is not compatible with any new motherboard.