Any Core2Duo will be better than that Pentium D. If you can get quad core like the Q8xxx or Q9xxx series and then overclock it, then with the GTS450 you can play most newer games on lower resolutions. LGA 775 is really a dead socket at this point but some of the chips still hold up well especially overclocked.
The GTS450 is not a hard core gaming card it is really a budget card and when combined with an older Core2 chip would be good for 1440x900 or 1280x1024 resolutions. I really do not see playing at 1920x1080 or above with newer games on anything but low settings though.