The graphics card is a nice one and yes you will notice a difference. Though the processor may be a bit of a bottle neck.
I suspect as long as you have a decent 350w PSU, or greater, you should have no problems.
Though I did once myself have a problem with an older motherboard not being able to provide enough power through the AGP socket. Probably not an issue for you, as this like a lot of cards these days (well AGP versions at least) require a separate power connection.
I'd say you should be fine.