I bought a X2 3800+ on Friday and a 165 Sunday. Both hit 2.4Ghz on stock volts. The 165 hit 2.5 as well, while the X2 didn't on stock settings. I didn't tweak any further, as all I wanted was 2.4 on stock settings. According to the simple Sandra sisoft benchmarks, the 165 at 2.4Ghz was faster than the X2. Temps were lower on the X2 by like 5-8C. I would have stuck with the X2, but it was a hassle to return the 165 as it was purchased online, where as the X2 was easily returnable at Frys.
If its retail X2 vs OEM 165 at the same price, go X2. They seem to run cooler, and OC well enough on stock volts should you chose to give yourself a free speed increase in the future. Adding the heatsink and fan, and possibly more fans for cooling make the 165 exspensive for basically the same performance.