I have a geForce 2MX in my machine that runs with just the stock passive heatsink. Mine is overclocked as well, with no stability problems. And the stock heatsink is fairly puny.
The MX is card on the far left. You can see the thin blue heatsink on it. The Geforce2 Ti is on the far right. You can make out alittle of its tall black heatsink under the Papst. The thin dark blue sinks on that card are memory coolers.
There are a couple more pics of it in my article: Rusty's Quiet In-Desk PC
My advice would be to download 3DMark2001 and run it. That benchmark will tax your card harder than you are likely to ever do. If it runs fine through that, you're safe.