Buying an older version than the current one may not be advisable anyway, so CS5.5 would be it, I guess.
But supposedly CS6 will be published in 2012, so you might want to look into that and decide for yourself if delaying the investment might be prudent or if you want to use the subscription model.
And another thing: A title like »Photoshop« in a Photoshop related Forum is pretty non-descriptive, so you may want to elaborate a bit more in the future.
Have you considered just downloading the free trial and trying it for yourself? That's what most everyone else does. You get 30 days to find out whether YOU feel it runs satisfactorily for you on your system. No one else can judge that.
Follow the "Try" link on this page: http://www.adobe.com/products/photoshopextended.html
Well, I would consider this a non-question. Officially you can only ever buy the latest current version, which is CS5.5, and it's the only one Adobe officially supports. That has always been their policy. The rest really doesn't matter. You could try and get a used copy of CS4 or such from somewhere, but since you will have to fill out a license transfer paper and really do not know about the source of the software, you may get yourself into a lot of trouble for nothing. And in light of the recently announced change in Adobe's upgrade policies it won't do you any good, eitehr. You'll may never be able to upgrade it and have to buy a new full version one day. So do the only sane thing: Download the trial as Noel suggested, then buy from Adobe or an authorised reseller or get onto the subscription program.