Okay, so your only option is to overclock with the FSB. The best way to do it is to up the FSB speed little by little (something like 5MHz increments) and see if you can boot into the OS. Eventually you'll get to the point where you can't boot into the OS without getting weird errors, or your computer may not even POST. When this happens, you've hit the limit and can either back the FSB down a bit or up the cpu core voltage and try to go higher. Once you're happy with the overclock, boot into the OS and run some benchmarks and stress tests; Prime95, 3DMark2001, and SiSoft Sandra are popular ones. If the programs crash then your system isn't stable and you need to back the overclock down a bit.
That's a brief summary, but there are many other things to consider when you overclock, e.g. PCI/AGP bus speeds, I/O voltages, Memory bus/FSB ratios, etc. I suggest you read through some of the threads in the overclocking forum until you're more comfortable with this stuff.
Most people have been able to get their 1.6A's to somewhere around 2.3-2.4GHz. Good luck and don't forget to keep an eye on those temps.