Based on its ancestry (i.e. Radeon 9000, re-vamped and scaled down 8500), I'd say a GF3 Ti200 would probably be the closest comparable. In terms of modern cards, say around the level of a really good GF4 MX or a decent 5200 128MB (without AA/AF enabled).
Can't be more exact, since the speeds and quality of those Nvidia cards can vary considerably. For example, I've seen memory bandwith on GF4MXs vary from 6.4 GB/s down to as low 1.3 GB/s (!), and all indications are that the 5200s have similar issues (especially on the crippled 64MB versions). Make sure you stick with a reputable vendor (and 128MB versions) if you go the Nvidia route.