If that gf card is AGP 8X only, then it will fly about as well as a lead brick. People will only be getting it inside systems, and will not be upgrading what they have. IF it will run equally well inside an AGP 2x/4X slot, then it has a chance.
How many people (besides the reaking rich) can justify getting a new system JUST for a video card?? Better still, to get a few more fps out of a game? I have an ATI Radeon 8500, 64MB version, inside my game peecee because it was much cheaper then gf cards that performed as well. I was tempted to replace it with the 128MB version, but thought about it for a few and realized a few things. One, games won't require that much video card memory for some years now, and two... what would I do with my 64MB card???
I firmly believe that ATI cards are a better product. Maybe my feelings are based on the fact that ATI chips have been used in Mac systems for ages. That kind of loyalty earns it a special place in line for my hard earned money. nVidia is too new to the Mac scene, so I feel no loyalty to them, or from them to Apple.
I have to wonder when ATI will be releasing their next generation of GPU. ATI's 8500 chip pushed nVidia to develop the gf4ti just to keep up. I remember that ATI had dual head video cards before nVidia did, and for less $$ too (standard issue with ATI cards, more recently added to ones with nVidia gpu's).
Personally, I see the current cards from ATI being able to handle anything you can toss at them for the next few years. I have no intention of going to a new video card until AMD comes out with their next generation chip. Then I will wait for the motherboard to come out that I want (on board RAID controller). At that point, I MIGHT make a new system and get a top end video card (95% sure it will be from ATI).
Bottom line, unless there is a game either out, or soon to be out (less then a month away) that REQUIRES (not recommends) a higher video card, keep what you have. If it comes down to that your software is dragging because of your video card... well, you have bigger problems then that since your system is probably a generation or two old anyway.