First thing you need to do is google the model number of that Asus mobo and get proper specs- the thing might not even run your processor! Then you need to test it with the sticks you already have. After that:
2 gigs of ram vs. 1 will improve your
BF2 game.
2 1-gig sticks will generally work better- you are correct that some boards do not like 4 sticks, and if you are overclocking (I am assuming that you are not, though) it becomes more relevant. If the cost is the same/nearly so then get 2 one gig sticks- this will almost certainly create a more stable system.
The important thing about memory (in almost any system) is to keep the sticks the same if possible, and the same specs if you can't find the same sticks. *** I do not have personal experience with that processor, but DDR sticks/Motherboard combos can be finicky.
As for your max memory, 2 gig is really all you need for your machine- chances are that your OS can't handle more than 3.4gig or so anyway (check into 32bit vs 64bit OS.)
Don't worry too much about the memory speeds of 4 vs. 2 and all that- you won't notice. Find your specs, see what hardware you have that works, and let us know. We'll point you better then.
