How to overclock ddr1 ram?

You don't really overclock RAM.

Generally, the RAM should run at the same frequency as the CPU. So if your Celeron runs at 100MHz (400MHz FSB), the RAM should be clocked at 100MHz (DDR200). If it's the 133MHz (533MHz FSB) version, the RAM should be clocked at 133MHz (DDR266).

RAM can be separate from the CPUs BUS speed. Generally you can select the RAM speed in the BIOS for SDRAM, DDR,2,3. ie. 200, 266, 333, or 400 for DDR. However, some bios's dont allow it and most of the time, if say, you buy DDR333 RAM, than the max you can run will be about 333mhz. Sometimes you can clock them higher, but not very often. Places like http://digi4me.com sell memory modules that are certified that they can be overclocked.

Just to clarify the above...you can run the RAM at a higher frequency than the CPU, but that is not the optimal configuration. A CPU:DRAM ratio of 1:1 is what's generally recommended. If you want better performance out of your system, add more RAM. 256MB simply isn't enough these days.