I just found this on eBay while searching for a Radeon 7500, which doesn't seem to be available in a Mac variant:
https://www.ebay.com/itm/251126702780?hash=item3a785162bc:g:F3UAAOSwh1haBsn4
It's a Radeon 7000 that is supposedly G4 Cube compatible for $25 shipped. I don't love that its description states that sleep mode functionality cannot be guaranteed, but otherwise, this seems like it would be a step-up from the stock GPU.
Maybe a little late to comment on this, but that listing is deeply suspicious, in so far as it seems to be implying it's some kind of official "Mac Edition" Radeon card. I would bet a million InternetBux (meaning, I dunno, a shiny new nickel) that it's actually a very, very low-end OEM PC card flashed with a hacked firmware. The real Mac Edition Radeon 7000 had both VGA and DVI ports on it, as did almost every "retail" PC Radeon 7000 card I recall seeing back in the day. Heck, even the PCI Radeon 7000 that Apple OEM'ed to stick in the low-end configurations of the Xserve used a PCB board that had the space for the DVI port, even though they didn't populate it. Low-profile Radeon cards like the one in that listing were almost exclusively the domain of companies like Dell that sold business-class PCs (like the Optiplex line) that included low-profile case versions. (The same card would show up in the "regular" versions too, just with a different bracket.)
Anyway, my vague recollection is that with some of the Radeon cards the Mac editions had larger flash ROM chips than the PC versions, so you couldn't flash the PC version with the exact ROM image used by the Mac cards; ROM hackers came up with "lite" versions that would fit without replacing the flash memory but the hack versions would have issues like, well, sleep not working.
Also, FWIW, I've moaned about this before but I'll repeat it: in terms of raw 3D performance the Radeon 7000 is barely a step up from the Rage 128, in a game like Quake it benchmarks like 50% faster at best, which makes it almost a pointless upgrade for OS 9 (if that matters). It might vaguely be worth it for OS X to get Quartz Extreme support, but that's about it.
(I *think* a GeForce 6200 is also mostly useless for OS 9? For an OS X only machine it might be a good choice.)