Like the others said, your CPU is your biggest hold back. Though DDR2 probably is a bit too. The graphics card isn't bad. I'm sure everyone would agree if you can afford it that a CPU, motherboard, and RAM upgrade is your best option.
However if you have access to a super cheap copy of Windows 7, it couldn't hurt. Windows 7 performs a lot better than Windows Vista. It eats up a lot of resources in general, but theres no telling how much that would help. Probably only get 1-2FPS improvement from a lower CPU use. So unless you have a really cheap Windows 7 copy available, or you are getting a license you can use again when you build a new computer its best not to worry about changing OS. However if you do plan an upgrade in the next year or so, you might want to consider it if it can be reused, because you will need it when you build the new computer anyways.