Can I hit 2.0 Ghz and how would I do it

I just overclocked my Athlon XP 2000 + over my 1.8 shown in my sig I got to 1.9 and it wouldn't go through the mem test all the way is this as far as I can go or should I unlock it even though I'm afraid to and then be able to change the multipliers and rock the house on this thing or is it just that my RAM can't deal with it I had the voltage to the ram at 2.7 at the time and the settings I believe are at the most aggresive. I would rather have the FSB higher for the Ram but I wanna see how High I can get this proc to go

If you want to see how hight he cpu can go then set the fsb to 133 and unlock the cpu so that you can change the multiplier to 12+ (if mobo supports it).... I don't think your cpu will be able to do 2.0 though, but it's still worth a try! good luck!

what would prevent it from going 2.0 If I'm able to efficiently cool it

It is just luck of the draw. Some chips will go higher than others, and there isn't a way to tell in advance what a given chip will do. I have an AGOIA Y (an exellent stepping by most accounts) and I can't get this thing stable (Prime95 stable) above 1.8ghz watercooled (38C at full load.) By all means go ahead and unlock it, but just because your chip runs at 1.9 is no guarantee that it will run at 1.91 (although it probably will).

what is preventing it from going higher than it is now is it the PCI clock? I know if that gets too high it affects everything so thats why I would need to change the multipliers right?

That is probably what is going on in your case. Your ram, hd, vid card or something has finally hit the wall. There is really no telling how close your cpu is (or your cpu might have hit the wall also.) Beezee's suggestion will help you find the culprit.