Wow, blast from the past.
Are we talking a D915GAV or a GAG?
If you could upgrade to a core 2 processor, I’d do that, but your buss speed is going to be a serious choke point.
You are limited to DDR 400/333 memory
If you have the GAV you only have a 10/100 NIC
At this point you need to seriously consider upgrading your motherboard, memory and processor to a recent chipset.
If your budget is limited you can still get a good stopgap with this motherboard:http://www.newegg.com/Product/Product.aspx?Item=N82E16813157359
You keep your 775 processor but upgrade G41 which gives you DDR3, PCIe 2.1 and a modern NIC.
Now the GT520. This is not a top drawer GPU. Not close. You will have to lower your gaming expectations a bit.
If its a budget issue I’d get the new motherboard I recc’d, 8gb of DDr3 1333 and a HD 7750 GPU. You will see across the board improvement, especially if you go to a core 2 processor.