need advice

Hi, I am running a mac pro dual core 2 x 2.66 and Pro Tools LE. 7.4 w/ 4 gigs of ram. (4) 512 ram chips and (2) 1 gig ram chips. I keep getting "increase buffer size" and "cannot get audio fast enough" messages. I recently installed some new plug-ins. I suspect I need more ram. How much ram do I really need? I was told by a company that I should get two sets of four ram chips to get optimal results. For example: (4) 1 gig ram chips and (4) 2 gig ram chips gives me 12 total gigs of ram. Is that too much ram? I now have two empty ram slots. What is the best way to configure my ram? I would like to just add (2) more 2 gig chips or 2 more 4 gig chips. The salesman had my head spinning on "getting two groups of 4". Does anyone know about this sort of thing? How much ram do I need and is it true about having the two sets of four ram chips?.......thanks.........

 

need advice

yes, I have increased the buffer size all the way up to 90% which seems high to me. I usually get the error message:
"You are running out of CPU power. Remove some RTAS plug-ins or increase Usage limit in the Playback Engine dialog. (-9128)" This happens right when I first start to work. Playback stops the first couple of trys and then it doesn't give the error message any more unless I put too many plug-ins on board. Iuse: lounge lizard, ultra analog, blue, battery, and others of that nature. Does this seem like a memory issue? Thanks......