Macbook pro getting bogged down

I have 4 gig of ram on my 2.66 ghz I7, 17" and when I have a couple of programs going or windows browsing, I notice it dosnt seem to take much to sloe the response down.. I am always waiting for things to happen, etc.
Now if its a memory problem then when I close programs do I not get the response back to normal like I should..

Should I get more ram and if so what should I get at the best price?