The most modern CPUs for LGA775 are the Yorkfield based Core 2 Quads eg. Q9350, Q9550, QX9770. Do keep in mind that these CPUs are now 10 years old, and outside of basic tasks are really starting to show their age. If you are looking at playing certain modern AAA titles some won't even launch because the Core 2 Quads don't have support for modern instruction sets like SSE4.1 or AVX.
At this point if you still have an LGA775 system and want to do anything with it beyond basic web browsing, video playback and office tasks, it's time to look at an entirely new system, the platform is simply obsolete at this point.