Teresa,
I have a Nvidia 6800 GT which is just a hair less powerful than the ATI 800xt on a 30" and Doom 3 runs very well.
When game developers make a engine, they usually push the limits of the available hardware because they resell the engine to other game developers.
So by the time they develop their games the hardware has caught up some and this gives the second game a chance at the market.
Doom3 will push every available piece of computer hardware in existence because it's relatively new, so everyone will have to turn down the options from maximum.
Doom3 will run smoothly on your machine, no problem. You will need to turn down some options, but the game is incredible.
Just a warning, Doom3 is a rather frightening and stressful game, in a story like fashion. You'll play it though and might never play it again.
A good game to play is Unreal Tournament 2004.
Once installed you should download the update from Unreal.com and install bonus maps that will give hundreds of different worlds to play in.
To do this you download the maps and control click on the UT2004 application icon and "open package contents" (the app is actually a folder)
From the unzipped downloads you just place what is what folders into the matching folders in the window you "opened package contents" of.
Then you start UT2004 and enjoy, at least this is a game that keeps on giving. There are hundreds of thousands of maps for all the different game types in UT2004.
