|"is a intel dual core 2.8 Ghz...."|
It would help to know which CPU. Is it a Pentium 4, Celeron, Dual Core Pentium, Core 2 Duo?
"better than amd semeprone 145"
AMD & Intel CPUs use different sockets, so if you have a motherboard that's socketed for Intel CPUs, you can NOT install a Sempron. Or if you have an AMD socketd board, you can't install an Intel CPU. The Sempron is a single core CPU so the Intel is probably better but it's tough to give an accurate answer when we have no idea which 2.8GHz Intel CPU you're asking about.
"should i change my graphic card to nvidia geforce 7800 or nvdia 200 series or radeon 3000 series"
Again, we have no idea what video you currently have, which motherboard, whether the board has a PCI-express slot or AGP, & whether you have a power supply capable of supporting a gaming card. You also need more RAM, 2GB isn't enough for gaming.
This chart should help with the graphics card choice:
EDIT: I just saw the Geforce 6150SE/nForce430 in your tags so I'm guessing you have an AMD based system. Is it a custom build or OEM (HP, eMachines, Gateway, etc)?