I also have an [original] 733 MHz Quicksilver (2001), but I've upgraded since owning this machine in Nov. 2001.
The original specs of this machine are as follows:
- 733 MHz processor, 256K L2 cache
- 40 GB HD
- 12x 10x 32x CD-RW
- 32 MB Nvidia GeForce 2 MX
- 384 MB SDRAM (got extra RAM as a purchase deal)
I've made the following upgrades within the past year or so:
- Motorola 1.0 GHz 7455 cpu tested at 1.467 GHz (purchased from GigaDesigns, currently running at 1.33 GHz, no L3 cache

which is why it cost me $250 instead of $400-450)
- 40GB, 120GB HD
- 8x/40x 4x/24x 12x/40x DVD+-RW(+R Double Layer)/CD-RW Sony DRU-700A (downloaded PatchBurn for system to recognize the drive)
- 128 MB ATi Radeon 9000 Pro Mac Edition
- 896 MB SDRAM
I had, and still have, the Unreal Tournament 2004 demo before I purchased the vid card, and I've noticed a slight performace increase while playing the game. The extra 512 MB RAM may have contributed, but going from 32 MB to 128 MB of video RAM is a big leap. I also have World of Warcraft (PM me if you're on the Gilneas server) and with my current config, the game runs well. I would have liked to get the Radeon 9800, but I just couldn't afford it at the time. I would agree with was said in the earlier posts; this machine just wouldn't be able to take FULL advantage of the card's power. Extra RAM DOES help a lot overall, but in general, the real bottleneck (as far as I understand) is the slow 133MHz front side bus.
To be honest, if you can, just save your money for a G5. Personally, my computer can handle the graphics and production work I can throw at it, and I could see myself using it to capability for another two years. To get back on topic though, if you're looking for a decent video card upgrade and don't want to break your wallet, you can't go wrong with the 9000 Pro. If you're a big gamer and want to see your games in all their graphics glory, go for the 9600 or 9800. You might as well pick up another 512 MB or 1 GB of RAM just to boost overall performance.
I'd rather spend $500 upgrading my Mac than spend $500 for a brand new top-of-the-line PC.