Right now I added a hitachi travelstar 7k200 and made my macbook pretty fast, but I am running my macbook with my 23" ACD and have 4 spaces open but when I change between spaces the graphics seems to be a bit slow.. will adding 4gb of RAM help?
Probably. Although I wish I knew what processor you have so I can give you the exact answer. RAM will work the same whether it is 256mb, or 4gb, it matter how good your processor is, and how much RAM your applications use up.
EDIT:
Oh holy crap, I just read your SIG. And no I don't think adding more RAM will help when you have 16gb. That is over the limit, if you are opening enough applications to make you even consider you are using up 16gb of RAM, then your processor is probably on the fritz.
I suggest just waiting for the new Macbook to come out, and then pimp that out like you did with your current one.
(By the way if I knew where you lived, I would break into your house, kill you, and steal your MacBook Pro. NO REGRETS!)
But you'll get a 2.4 MBP one of these days anyways because....because...i don't know you'll have one of your reasons..Spaces is going to scream on the MBP, but it will scream even more so on a 2.5 MBP....only to be surpassed by the 2.6 MBP, let alone a 17"! At the time you're thinking, you're there! Boy, are you not...but hey, you can always jump on the MP wagon! 100 threads in the meantime, preferably 1 every 5 minutes to keep us posted about your ever changing ideas and views! But hey, it's ok, as long as you're not manipulative...i mean i don't think you are...
The good thing is, we'll always be able to revel in your sig that ruthlessly clarifies, that without a doubt you were once a MP power user! Even if you're on a MB! I love an informative fellow! And of course you've got to admire the endless enthusiasm without proper knowledge!
Never mind, just the observer in me!