I have a Leadtek MX 440 - the best overclocking card I have ever owned!
Using just the Winfast utility supplied with the card, I took the core to 325 and the mem to 450. No problems, no crashes, nothing. After running the card at these settings for a couple of months, I replaced the HSF with an old P3 server heatsink, attaching the card's old fan. I added my own ramsinks bought from an electronics store. The Leadtek MX440 ram is rated at 4 ns, which gives you a max frequency of 250 single channel, thus 500MHz DDR. Sweet! I took the mem up to 500 and the core to 375. Remember, the stock settings are 275 core and 405 mem! The card runs just fine. If you increase either the core or the mem, artifacts appear, otherwise no problems. The card has been running at these settings for four months now. No problems.
Leadtek is the way to go for OC'ing 3D cards - they rate their components way above spec.