Incorrect.
Because on a PC you have adjustable resolution, something like 1024x768 can be done on a PC. On a 360 you are at 1920x1080, you aren't going to get a PC like yours to do for the reasons I outlined above.
Now if you didn't want the 1920x1080 true HD that the 360 is doing and you just want the game to look good, then it may be possible with a high end AGP cards by dropping the resolution significantly.
Like I said from the start though, you can't get Xbox 360 quality graphics out of your pc. It would take significant upgrades to be able to come close, and even then you are trying to overcome games that are specifically coded to run as perfect as possible on the 360s hardware. The PC versions aren't specifically coded to 1 exact set of hardware, they have to run on a variety of hardware.
I feel like I'm rambling, but I also get the feeling you are not entirely grasping why we are saying you can't do it, even with a new video card and more RAM.