what is total available graphics memory?

I've been looking at different models of vaio z and different models have different total available graphics memory. Does this make a big difference (790 vs 1300mb for the intel graphics, 860 vs 1390 for the nvidia)?? I read the footnote on the sonystyle website but I am still confused.

Example:
"NVIDIA® GeForce® 9300M GS GPU with Total Available Graphics Memory of 1370MB (max.) and Mobile Intel® Graphics Media Accelerator 4500MHD with Intel® Clear Video Technology with Total Available Graphics Memory: 1300MB (max.)"

Thanks

 

The lower res Z has Nvidia vRam of 128mb

The higher res Z (z590) has 256mb vRam

Its the onboard memory on 9300m GS

 

also, the max amount mention on the web is achieve by sharing the dedicated memory with ram memory (eg: nvidia turbocache etc) provided you have sufficient ram memory.

for example, using 512mb ram will have 64mb intel gma950 memory, while adding to max ram, the configuration of available memory on intel gma950 will be change to its max, which is 128mb

i dont know if you can change the memory allocation in bios, but i cant change mine. so it will be automatically allocate the memory depending to the amount of ram memory i have

like sonoritygenius said, the dedicated memory one nvidia hold is 128mb and 256mb without sharing with ram.