8GB should be fine for stills provided you aren't going OTT with enormous stitched images or anything crazy like that, though obviously more RAM is better. Do check out Apple store refurbs as you could save some money. Also consider a Mac mini + quality monitor if you don't actually HAVE to take the computer with you - editing on a small screen is possible but it sucks.
Spec wise, LR and PS are very processor intensive, so the fastest processor you can afford will help, however they (mostly) don't benefit from graphics acceleration in their present form and LR actually runs better with GA turned off. Because Macbooks now cannot be upgraded you should plan to buy the spec you will need in a couple of years time, rather than the minimum that will work OK now.
