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I am guessing it is about a 3 year old notebook. I would also guess that you would find CS5 totally unacceptable on this under powered (for CS5) dual-core CPU, with only 2 GB of RAM, probably only a 5400 rpm single disk drive and no way to use the Mercury Playback Engine. If CS3 is doing your applications stay with it until you can get a more appropriate platform. CS5 requires lots of CPU power, lots of RAM, at least two 7200 rpm disk drives and to take full advantage of the MPE right now you really need a desktop.
Sorry for the bad news but we would not want you to invest in CS5 and be disappointed.
A remarkably graceful delivery of the obvious answer.
Bill is a kind and gentle soul who shares his knowledge freely and otherwise gives to this community. He is the force behind the premiere benchmark (and so one of the best authorities on what is underpowered). He has now taken on a junior colleague (heh, heh; neither Bill nor Harm are spring chickens) to assist him in the latest offering.
You should fully reward him by giving double points for a correct answer, but, alas, you can but mark it answered and he will get a dazzling 10 points. Only moderators can claim to give millions of points for various feats of daring.
BTW, don't forget that PR CS4 is included in the CS5 suite. I upgraded to CS4 from CS3 at the perfect moment to get CS5 included. I finally installed CS4 to test an idea, and find that it runs about the same (for SD only material of course) as my even-more-underpowered-than-your-laptop does with CS3.