First of all, as molly has stated earlier, run the Memory Advisor tool to make sure that your mother board can even support 2gb sticks of memory (some motherboards can not).
***If you don't care about technicals, please skip to the last paragraph.
Next, look at your motherboard, do you see 2 pairs of coloured slots for your ram (we are assuming that there are 4 slots, if there are only two then your out of luck already)? If there are, that means your motherboard is capable of running duel channel ram, this is where you will need to determine what kind of performance you want and which ram to buy.
Now there are two things to look at, practical performance and cost.
Ill answer your clock rating question first. As for which to buy, 800 mhz or 667mhz, it wont matter interims of performance. Your system will run the ram at the lowest common speed (in this case, 667Mhz). However, 800Mhz ram is much cheaper than 667Mhz (if you buy online). 667Mhz is dated technology, thus a bit less non standard, 800Mhz is more standard now (which means its produced more and cheaper). Buy 800Mhz.
RAM can run under duel channel mode. However, depending on how detailed orientated you are, it may or may not matter to you. 1 pair (2 sticks) runs faster than 2 pairs (4 sticks). Less large capacity chips run faster than many smaller capacity chips.
Lastly, 2GB sticks are cheaper than the equivalent in 1GB sticks by 10-20%.
If you want the absolute best performance, buy a pair of 2gb 800Mhz sticks and throw out the old 1gb sticks. If you want to go with a balance of cost and performance, buy 2 1GB 800Mhz sticks. If you just want low cost and could really care less about performance, buy a single 2GB 800Mhz stick.