Patrick,
Let me go over a few things with you, forgive me if this is stuff you already know.
To start with, something like the 8800GTX is totally out of question for several reasons. Your system uses an AGP card. This refers to the type of card "plug in" your system needs. Several years ago AGP was top of the line, but that has since changed. They have now "upgraded" computer hardware to require a different plug in for cards called PCI express. They did this to create a newer system that can handle more info transfer than what AGP could. AGP is fast falling out of availability. Some are still producing cards because there are alot of people that haven't purchased newer systems and still want to upgrade their equipment, but over some more time it will be discarded altogether.
In some ways this is very beneficial to you right now. Since you dont really want to play games and so forth, its not totally necessary for you to pay for the best AGP card you can get your hands on, but on the other hand if you want your system to still be able to handle things like the aquarium and if you ever see a game you want to try then you will need something more than what you currently have. The good part is that AGP cards arent that expensive any more. If $200 is your limit, the good news is that you can get an AGP card in the 7xx series, which is one of the top of the line cards left in AGP for between 100 - 140. Less than what you were expecting.
Something like the 8800GTX card is $500+ depending on the version picked, but they dont make those in AGP so it wouldn't do you any good anyway. Any card in the 8xxx series will not work in your system. Something in the 6xxx or 7xxx series in an AGP card will.
Just for discussions sake look
here. You can see a 7600GS card for 139.00 from newegg, and read the reviews about that card.