It would speed things up slightly.
But the main reason why a dedicated graphics card has its own Gddr 5 ram on the card it`s self is to do with
Bandwidth or how wide the data bus is in bit`s between the Gpu and the memory situated on the card.
The fatter the pipe, or data bus in numbers the more data you can pass through the bus, in a shorter amount of time,one data pass, cycle or refresh, at a larger volume of data.
When you use on board system memory, for example DDR 3 @ any speed.
And for example a APU or cpu with a Gpu solution, the problem is how wide the data bus is.
Depending on the motherboard and the config of DDR 3 main system memory, amount of ram sticks and memory pairing modes.
64 bit`s in single channel mode
128...