In Fallout 3 I would have no idea. In Fallout New Vegas however it's easier to RP an Ex-Enclave. Depending on your run, I would say:
[*]Depending on the affiliation of your character, I would still say that helping the NCR or Brotherhood requires a tremendous effort because of what they did to the Enclave. I personally go out of my way to kill everything involved with both organisations but I'm a fanatic
. Try to be more selfish in the wasteland, maybe keep to smaller settlements or even bunkers to remind you of home.
[*]If you playing an Ex-Soldier then go for Energy Weapons/Guns, Medicine and Repair. Combat, first-aid and weapon maintenance, makes sense for well trained soldiers no?
[*]If your roleplaying an ex-civie then go for Science, Medicine, Repair and Energy Weapons - rudimentary weapon training is required even by civies and they have energy pistols, so it makes sense that civies would know a bit about EWs of no other weapon types.
[*]Always have a fairly good intelligence but a slightly lowered charisma - nobody in the Enclave is really accustomed to life in the wasteland and dealing with the outsiders, they'll be obviously dis-trusting and have really lived in a militarised, regimented society. In short don't pick speech. Nor Barter either as the Enclave society didn't really allow for such concepts, being so controlled.