Welcome to GeeksToGo,
My name is Cbarnard I hope I may be of some assistance to you.
I understand you are trying to upgrade this system with the least amount of money, But parts for older systems are more expensive to purchase. Usually, but not always you can replace your motherboard, CPU, and Ram, for the same price to upgrade. You may pay a little more for the new parts, but you won't be stuck in the 19th century. I bet for $200 I could get you set up with a new mobo, cpu, and ram. And it will blow your system away, all for $30 more than the upgrade would cost click here
for a better example. Also I researched this info so its accurate within $10
I just worked up a build for you $206 up front with $30 in rebates
You will probably also need a new powersupply that will be an additional 30-40 dollars.
But if you are still set on just upgrading I will be happy to assist you, I just don't want you to make a uninformed decision. Most people don't realize its cheaper to build this way than upgrade... But I have armed you with enough for the decision you will have to make.
NVIDIA GeForce FX 5200 128Mb Graphics Card (Upgrade Possible)
this is the best you are going to do with your setup, $99
512Mb RAM (Upgrade Possible)
that is the fastest ram and the most you can install in your mobo $70