Well, it depends on your motherboard, how much FSB it can handle and how many ram slots it has for you to reach 4gigs and the processor you have on your computer. If you have a 64bit processor then when you hitch on those 4gigs, you might not have such a hard time, chances are that it will just pick those up and be on it's way. On a 32bit proc though, that might be a little different, depending on if your board comes with a decent memroy index. Usually when you install brand new ram for the first time, it get indexed and sorted sort of like how on a MAC, the HD gets indexed, they call it journaling and it just basically means that it's tallying up the ram for faster use. Well if you have a nice special board it will index all 4096 of your ram, otherwise it might tell you, you have somewhere between the 3 to 3.5 range of ram when you really have 4 gigs, because it takes into account your video ram and it's limit of index and registrar is 4Gigs regardless.

It doesn't quite mean that you arn't using all 4gigs upon boot it just means that the computer doesn't know it's own strength type of thing. Technically it should use all 4 gigs but it just won't have the entire thing registered, and it might take a performance hit since it would have to resync those sticks in the bios everytime it's cycled, theoretically anyway. In any case I would go just up to 3, just to be safe, however if your willing to try it out and then if it doesn't work out, you can just return it; and just have 3 gigs. 533mhz is decent but if you have the monies, I would find out how much FSB my motherboard can do and purchase your ram accordingly.

If you bought your computer from like say Dell or some of these major computer retailers, then you can find out what kind of motherboard you have just from the model number infront of the case. The difference is not alot but somewhat significant from lets say a 533 and a 667 but still there's something that would help you computer be faster. If that is stock RAM, then chances are you probably have room for improvement. You can overclock your FSB speeds by upchanging the modules on your board with the right Bios and you could go with even 800 if you do it right but overclocking anything to it's limits usually means less longevity.

And as for the mixing of RAM sticks, I wouldn't have two different sticks on the same channel on your board because your not using it's full potential. Let's say you have a 677, on one and an 800 on the other on the same channel, the bios will configure itself to the slower stick as default and your faster stick will take a performance hit, so ideally it's better to have identical sticks that are at the fastest speeds without stepping outside the bounds of compatability. But you can atleast on recent motherboards to date. Back in the day motherboards wouldn't boot up at all because the bios had no revert compatiblity when presented with offchanging sticks but that should no longer be the case and from the speed of the stock ram that you have on your computer I'd say your computer is fairly recent, atleast not older then 4 years, so that shouldn't be an issue. The Extreme business, the only thing that I could think of as to why one would be extreme and the other not is that one is manufacturedly overclocked, which makes no difference but your bios will default it's settings to the non-overclocked stick. So it will work just you just won't be as Extreme. lol But yeah you can mix different speeds and timings on most motherboards just your operating at the slower rate..