To me personally the Radeon 7500 AGP Mac Edition is the best choice when considering the relationship between cost and performance. It fits into the cube just fine and you won't have to do any modifications to either the cube or the card other than removing the slot cover since it won't align with the Rage 128's fencing. I got mine off eBay for 20,-- so that's still feasible (unlike e.g. the Geforce MX/4 or whatever that thing is called exactly). Runs beautifully with full driver support under both OS 9.2.2 and OS X 10.5.8 at 1080p (granted I am at max RAM and using a Sonnet 1.8 GHz CPU).

-- BeniD82