It depends on the motherboard you have. Name it, what model is it?
As I know e5450 Xeon is quite cool and have base clock of 3Mhz, and if you have good motherboard and aftermarket cooler, you can overclock it to 4Mgz or a bit less. X5460 is better in the part of stock clock and can be overclocked 4mhz and higher but again if you have good motherboard and good cooler, because this Xeon is much more hot than e5450.
Moving from Core2duo will show you improvements, but moving to at least second generation i5 will give you a lot more performance.
Anyway, look at EBay or Aliexpress, there are lot of these Xeons that already modded to fit your 775 socket. The only thing before installing Xeon is to watch your motherboard bios version, and find the last one mod bios to flash to your motherboard, so you can run Xeon. Not all motherboards can run Xeon at once, some need to be flashed.
Sorry for bad English.