Hey mate, here is a good guide for OCing, http://www.xbitlabs.com/articles/cpu/display/oc-guide.h...
it applies to opterons as well, though opterons have slightly lower voltages at stock.
Opterons are good overclockers, but it depends on the stepping on each CPU, some OC better than others. here http://wiki.extremeoverclocking.com/wiki/Opteron is a good guide to the steppings and what you can expect from each one.
If you want to find out more about your CPU download CPU-Z which gives you info on your CPU memory and motherboard.
As for going to 2.4 without changing the voltage, id say that that is unlikly, though if you have a good stepping it is certainly possible ;p
Anyways good luck with the OC.
wel 2.4 should be really easy, 2.7 is really easy too, just put your mobo at 300 FSB, and run your HTT at 3x with a small increase in NF4 voltage, and set mem to like 166 and you should be good to go. but 2.4 is easy, 2400/9 = 266 fsb you might be able to run HTT x4, and then reset your mem so its still DDR400.
mine went to 2.6 on stock voltage and runs 24/7 at 9x333 with 1.45v highest benchstable i have gotten so far is 9x345 with 1.58v and am waiting for winter to get some more. Anyhow with a decent (not great) air cooling like my 9700-cu you can safely up the voltage and htt with no worries.
Well here is something that may help some people still on the opteron fritz although I am sure everyone went core 2 recently. However, the new revisions of the Opterons family F have a static Tcasemax at 49C and a TDP of 35W which is inaccurate. I scoured the net to find a solution and am yet to find one. No one has any accurate data. I have a Opty 170 and wanted to OC it. I am running stable at 2.61 so far using a 9x multiplier and it never hits above 51/52 C with 2 instances of P95 running on it (1 each core). Its a great processor and crystal gcupid (think thats the last part, too lazy to check) says that it was a 30.4 % overclock which is crazy. I know that I could go higher but this is ok for me. Fx-60 speeds.....Hope it helped people.