Memory RAM upgrade

Hello,

I have a cheap Patriot 1gb memory DDR2 800 (PC2 6400) at 1.8 volts, with a cas latency 5 and timing: 5-5-5-12.
I can now afford to upgrade. I would like two more 1 gb sticks.
Is it better to buy the same model stick I already have or one with similar specifications? Can I buy a faster memory? Can I buy a memory with the same speed but that works at different voltages, or faster timings/latency?
Too many options that are confusing me.
Currently I am not overclocking my memory at all because it was a cheap stick and I am worried that it will easily overheat.
Thanks.