The GT240 would be very unlikely to meet whatever performance requirements you have, unless your resolution is low, like 1024x768.
Unfortunately, I would not trust a HEC "450W" PSU to power a GTS450. If your budget permits, replace your PSU. A quality, modern PSU has full range active PFC (no little voltage switch) and 80+ certification. Antec, Corsair, Seasonic, and Enermax are among the better brands.
If you do not have the option of replacing the PSU, then get a HD5670. It uses very little power, and will outperform a GT240.