It doesnt look like you can reuse any of the parts from your Current PC. (Besides the hard drive, and possibly optical drive)
In "the next few years" basically every current PC will be outdated for gaming. But if you wanted this PC to last for "a while" you will want a Quad Core CPU and a 8800series GPU. Which means your looking upward of about 1K$ for the Tower.
Tell us WHAT is included in your budget: Tower, Moniter, keyboard, speakers, etc?