here are some guidelines:
1) get the newest bios for your mobo. I bought an Abit AN8 32X (abits version of your board) and couldnt get past 279HTT, but now its stable way past that with a bios update.
2) drop the HTT link down to 4X. If you manage to get your CPU past ~300HTT, then you need to drop down to 3X. The HTT bus doesnt like to be running past around 1.2GHz in my experience. Some people even feel you shouldnt go past 1GHz, but I am not one of them.
3) your memory may crap out before the CPU hits the limit. so when you're trying figure out a max speed for the CPU, run a memory divider. since you have DDR500, the 333MHz divider will give you DDR500 at 300HTT, so you should be able to find the CPU's max with that. If not, then drop down to a 266MHz divider.
4) in order to get higher CPU speeds, you'll probably need to up the voltage some. the Zalman 9500 is a pretty damn good cooler, so I'd say anything up to about 1.525V should be fine. I'm running a Zalman 7700-AlCu on mine at 1.52V, and its hitting around 52 degrees C during load, which is acceptable. Just keep cranking up the HTT speed at a given voltage (start at stock) until its unstable, then bump the Vcore up by one notch (.025V). Repeat until a) youre satisfied with the speed b) you hit around 1.5V or c) you're getting really high load temps (high 50's, low 60's).
5) Once you have found the max CPU speed, drop down the CPU multiplier (to like 8X or 7X) and crank up the HTT to find the max memory clock. you may have to loosen the timings to like 3-4-4-8 or something in order to find the max speed.
6) once you've found the max speed of the CPU and RAM, you can set the appropriate divider that will allow you to maximize both as much as is possible. Also, if you're getting pretty good timings at a high speed, like 2.5-3-3-7 at DDR480 for example, then you might want to set your divider such that you can get these timings.
7) most importantly, TEST THE STABILITY AT YOUR SETTINGS!!!! What I do is this: I open superpi, and run the 1M test. Then i keep adjusting the HTT speed until superpi fails. Then I bump up the vcore and repeat. Once I'm around the maximum range I think i can get safely, and I've found a speed that is superpi stable, i bust out prime95. prime95 is the best tool, altough there is a better, tweaked version called Stress Prime 2004, or SP2004. It makes it much easier to test multiple cores, and has a nice GUI. Simply open up 2 instances of SP2004, set one to core 0 and one to core 1, and click start. One of the cores will probably fail after a few minutes. Then back off a few MHz and try again. Eventually you should get it to run stable for hours. Some people insist on testing for 24 hours, but I dont really have time for that. Its not like Ive ever been doing 2 different really intensive things on a PC for an entire day straight. For me, if I start it in the morning, and its stable when i get home from work, then its fine for me.