As long as it your motherboard has a PCI-E (that is an old graphics card you currently have), you should have no problem with that, and it will be a solid, futureproof investment. 9800s are great cards.
I would recommend you upgun your processor too at some point in the near future, along with the RAM and the GPU you're already looking to upgrade. It's by no means a bad processor, but you can pick up an X2 5200+ or probably even a 6200+ for like $35. Although not essential to your rig, you might end up with your processor being the weak link in an otherwise fairly decent chain down the line. Just make sure you buy a Socket AM2 processor (like the older X2s like I mentioned - I currently run a 5200+ and can max out pretty much every game I play, no problem. You can probably pick up better for less than I paid now tho). You can't have an Intel chip, either, as your mobo won't support it.
I apologise if I gave you a bunch of info you already knew and sounded patronising - I don't know your tech competency.
But yeah, the 9800's a great card, will serve you well against pretty much any game that currently exists.
I'm not sure if that's a little pricey considering the card's a couple of years old - I'd shop around for the same card elsewhere.