You really need to consider a new build altogether. Pentium Ds are slow CPUs, and will hold back any graphics card more powerful than your 9800GT. You can also probably use more RAM, some games will use 2GB all on their own, and your OS is going to be eating probably 1GB of that memory. At the very least, you'll need more RAM, and you will need to track down a fast Core 2 Duo or a Core 2 Quad that your motherboard supports in order to get a playable experience in Metro. Given the fact that the Core 2 CPUs are very pricey right now, it probably isn't worth going that route.
Metro is probably one of the most demanding games out there. There is no single GPU that can run it on ultra with all the DX11 features at any decent resolution and maintain playable framerates. Even at a really low resolution you would need a top of the line card to even have a chance of doing ultra and staying above 30FPS. Witcher 2 might be slightly more demanding if you turn ubersampling on, without ubersampling, Metro is more demanding.