The system you're running your Core 2 Duo in is probably too outdated to do the 750 Ti justice.
Keep in mind for a GTX 750 Ti you want at least:
A 400w PSU.
A motherboard with a PCI-E x16 2.0 slot.
An Intel i3, i5, i7 series CPU.
4GB or more of RAM.
Your GeForce GTX 750 Ti might run in your current system, but it won't be functioning at 100% of its capabilities unless you meet the minimum requirements I listed above.