I have a T-Bird 900 with a V3-3000 (sounds similar to what you would have if
you upgraded from the 450 to a 850) and I can even play N4 with 50-60 fps.
This will also allow you to use the speed difference of your processor for
other things as well besides games.
provided your ram is more than enough you should effectively concentrate on
the video, get a prophet II mx because if your video is weak you can get a
pentium 6 running at 5 gigs the bottleneck will be there all the time.
Think of it like when you buy car audio, if you buy a 300 W stereo system
it's better to buy 500W loudspeakers to always be able to get the max out of
the system, well same, you always want to get the max out of the system, not
being blocked by your video card.
The V3, while not state of the art, will get you by in most games.
-Larry