On stock cooling, I have my 3100 (a 2800 would've most likely done the same) at 313x8=2504 with 1.5v. Note: The s64 2800 goes up to 8x, the 3100 up to 9. I took my mult down a notch because my xp-120 isn't here yet. I idle at 24C and load at maybe 35C. Air temp is ~70F...so maybe 20C.
The palermo's have ungodly potential, you wouldn't believe. Mine has booted at 2700 (not that it doesn't any higher, just haven't tried) with 1.7v. It was running fairly hot so I took it back down. Just make sure you get a 256k version; your chances of getting an E6 stepping (aka uber-OC) will be greatly improved.
Remember, sempron 64s are basically Venices. You know what venice can do...only difference: single channel not dual (meh 1-2% loss there) and 256k not 512k (meh 2-10% loss there)
*Hoping for a snow day tomorrow, here in N. VA*