Welcome to the Cube club Bunsen! Mine is my primary Mac and I still think it's got a few good years left in it. Regarding suitable video cards, the Apple Radeon 7500 is a nice choice (ADC, DVI but no TV-output), while the flashed XFX "WANG" model Geforce 6200 256MB AGP is a pearler if you can find it (which I did ~ 18 months ago at the Box Hill computer swap meet, cost $90).

Keep in mind with any upgrade you will need to install a base fan, and I'd think those dual 533s would work quite nicely with a modded heatsink your Cube ... clocked to 600?

JB