You'd be going to at least the performance of a GF Ti4600, but with native DirectX 9 support. It's not just a little under the 9800 Pro, and there is a performance gap between them.

If you want the faster video card, you might see about getting a little meatier power supply from Shuttle for it if possible. I know they exist, but I don't know if they're available for the one you're looking to upgrade.