GPU - Old and outdated. Will run games fine, but don't expect wonders from it anymore. Its a re-branded 9800GTX+ which is a re-branded 8800GTX (almost), so its got the power of a high end card from 2006.
Power supply may be re-usable if your new system isn't overly demanding. Modern GPUs are rather power efficient. I'm running a GTX660Ti on a 650W, but it can run on a 600W or even a 550W if your other parts permit.
CPU cooler is basically useless if it is indeed a 775 based design. just because it fits the board doesn't mean it fits the CPU.
DVD drive is always reusable.
Monitor, ditto. Always reusable.
Accessories, once again, reusable.
If you are looking to reuse the GTS250, then stick with either a 775 quad core, or maybe an i3 or i5 quad. Its really hard to future proof a system using a 250 as its focal point. Anything you come up with will be behind the curve and be on the chopping block in 2-3 years tops.