It will be backwards compatible with your motherboard.
I would think it would be a pretty big upgrade.
WHat PSU do you have? I'm sorry if I missed it.
I'm sorry, I forgot to list my PSU. It's the stock PSU from Dell for the Dimension 8400. I believe 350 watt/28 amp. Even though it's old, I should still be okay, right?
Thank you all for the prompt and helpful responses. Pending any answer on my PSU, I will go for it and buy that GT240.
Again, many thanks to you all and your great, informative site!
EDIT: I went ahead and bought it. At worst, I can recoup that money from a friend, who would buy it off me if my PSU cannot handle it. With that rebate it comes to under $50. Low risk + high reward = No brainier. Thanks everyone!