Most 667/800 sticks will also have a 533 profile, they should work fine. Either the motherboard will upclock to meet the highest standard profile or drag them down.
Each slot is capable of holding a 2GB stick. The entire system should be capable of running 4GB.
In order for the system to operate in dual channel mode (twice the memory bandwidth at up to half the latency) you must have two memory sticks installed. Thus it is preferable to have 2 1GB sticks over a single 2GB stick but best of all would be 2 2GB sticks
I hope this answers your questions