I use the Thermaltake Volcano 6cu with an overclocked Athlon 1600xp. Do yourself a favour and buy some Arctic Silver II for the thermal paste, and take your time applying a smooth thin coat. Beware the clip on the 6cu, as it requires some force to put in place. Ensure you use a flat-end screwdriver that fits nicely, or you'll slip and drive it through the mobo
I actually used my large hex key to push on the horizontal part of the clip higher up, then used the screwdriver to push the clip in place.
The generic thermal paste with the 6cu on the 1600xp (o/c to 1512Mgtz) had a standing temperature of 38c, and under load a temp of 45c.
With the Arctic Silver these temps dropped by around 5c, so that now my average temp is 33c, and 38-40 under load.
Dans Data (www.dansdata.com) have reviewed this heatsink/fan well, suggesting it's big brother, the 6cu+, while it has a more powerful fan, doesn't lower temps much beyond the lower rpm fan of the 6cu, and is considerably noisier to boot. So for the price, and with some nice goop, this is a very nice choice imo.