There's an excellent page on Wikipedia that lists all Intel graphics products and their corresponding chipsets:
If you look for 945G you will see that this chipset uses the Intel GMA 950 motherboard-integrated graphics processor:
There are more specifications on the GMA 950 on Intel's web site:
If you want to be absolutely sure which graphics chip your system is using, you can check by PCI device ID. To do this in Windows, open Device Manager, expand the Display adapters section, right-click on the display device and choose Properties, go to the Details tab, choose Hardware Ids in the drop-down box, and look at the values. You should see one that starts with one of the following:
These device IDs are listed in that first Wikipedia page I linked to.