Onboard graphics built in with a motherboard do not come with memory of their own (there may be a very few exceptions). It uses a part of the system memory (the memory you install on the motherboard) as the graphics memory.
The amount of memory it can use as the graphics memory depends on the chipset. IIRC, the 845 chipset can use upto 8MB as the graphics memory. If you have 128MB total memory installed, you can allocate 8MB for the graphics, leaving 120MB for the system. If you set 1MB for graphics, you'll have 127MB for the rest of the system.
Note: I'm not too sure about the max graphics memory size for an 845. Somebody correct me if I'm wrong.