@Ragman,
Reality is that a varied healthful diet which has an effective anti-oxidant and increasing blood flow to the brain should help.
Frankly, rather than researching and taking a supplement like DHA-Omega 3, I'd rather get my omegas from eating organic cold water fish like sardines or salmon, which are rich in Omegas and other nutrients and micro-nutrients.
My take on this is a healthy lifestyle doing regular exercise and good nutrition (non-smoking) is wisest move rather than a questionable supplement. Doing regular (but a variety) of challenging activities that require using different pathways in the brain is known to help with memory and concentration.