ok, if I’m reading over my info correctly, the i845G can support DDR333 (aka PC2700) DDR memory.
However it also supports PC133, DDR200 (aka PC1600), and DDR266 (aka PC2100) memory (these are the officially supported modules).
In CPUZ, under the Memory tab, you’ll see a group labelled Modules Information, this will contain a combo-box with text like DIMM #0, DIMM #1, or somesuch, the text to the right of that will give you a brand, memory technology, speed, and size. If your computer is using single data rate memory (as opposed to DDR (Double Data Rate)) then you’ll absolutely find it worthwhile to upgrade to DDR.
Your board may not support DDR333, but it might. If it does not support DDR333, it will run those modules at DDR266 speeds, so you can still use the hardware. If you want to upgrade, I’d recommend that you get DDR333 (aka PC2700), but make your own descision based on price differences.
Since you have a single channel motherboard, I don’t think you’ll see a clear advantage to using 2 sticks instead of 1.
Note that if your current memory is PC133, then you can’t add a stick of DDR and expect it to work, in this case donate your memory to a friend, family member, or charity.
You won’t see a big advantage in using either 1 or 2 sticks of memory after you upgrade (as long as the prices are comparable) but if you plan on getting 2 sticks at one time, then buy the same brand, same speed, etc…
I’m sure I’m missing something in here, so feel free to ask me to clarify (I don’t want to feed you too much information either).