Your GPU is decent considering its age, except for the fact that it doesn't have its own on-board memory.
One of the consequences is that your GPU needs free RAM in order to work. Assuming your GPU needs the full 512 MB and the game itself needs 100MB, and rounding up to be safe, you will need to check Task Manager and make sure you have 700 MB of free RAM. On your system, that probably means no other programs should be running.
Another consequence is that your GPU's speed is bottlenecked by how fast your computer can move memory from the RAM stick to your GPU. This can really kill performance, and unfortunately there's no way around it.