Japamac's really the authority on video cards for these old Macs - you can't do better.
If I were staying on the cheap, I think I'd go for the GeForce 4 - there's a 64 MB variety with ADC and DVI connexions and a 32 MB one with ADC and VGA (I think I'm right on that). The Radeon 9800 is definitely better, and the nVidia 6800 is better yet. It's the one I use (on Japamac's recommendation). Both have DVI connexions and VGA (which usually works - there are occasional anomalies - pay attention to the seller's comments).
I've had good luck with eBay video cards. I like the Apple ADC monitors,too, but they are better purchased locally - where you can see them work and inspect the stand: they don't ship worth a darn.
Regards from the house of Too Many Macs... Jon
P.S. You might try Japamac's Radeon 9000. I just found one lurking in a dark corner, and plugged it in. Works pretty well, too. There are several varieties of them, too, with different port configurations, 64 or 128 MB, and prices from $25-75 on eBay.