Good question, I am interested to hear what people take as a vegan option. Here's a little more info if you are interested and my overall 2c on fish oil, how much to take, interaction with diet etc...
I would say 5g is a good starting point but I wouldn't be too afraid to go heavy with the EFA's, just tailor the dosage to meet your needs & requirements. It may sound a lot but 8-10g fish oil really isn't overly excessive IMHO, especially if you are using it treat something like depression, muscular inflammation or anything specific with regards to the cardiovascular system. I would consider a big dosage of Omega 3 to be around the 10-20g mark personally; I have worked alongside a top NZ track cycling coach that had his squad on well over 15,000mg Omega 3 a day with good results; the only common side effect can be an upset stomach, in which case you can just reduce the Omega 3 oils until you start to come right, much like you would do when you are running big doses of Vitamin C. Doesn't really matter whether each capsule contains 1,000mg or 2,000mg, it comes down to the DHA/EPA content (DHA is the important one, the higher the level the better).
Be aware of your diet in combination with running Omega 3's as it has been shown in studies that cholesterol-lowering diets can increase the risk of depression. This is due to the decrease in Omega 3 fatty acids which can reduce the DHA levels in the brain (the brain comprises of anywhere up to 50% DHA). DHA is an essential part of the neuronal cell membranes, but if it becomes replaced by Omega 6 fatty acids then the likelihood of depression increases as this can cause changes to the structure of the membranes.
Look for Omega 3's that have a good ratio of DHA to EPA, the common ratio tends to be 180 to 120 but you can get higher than that (usually oils contain higher levels and you do get what you pay for). I routinely take 18g and have no aches, pains or even DOMS. Mine is a liquid Omega 3 which gives 927mg DHA and 1,900mg EPA; a standard 1,000 mg fish oil capsule provides just 0.3 g of EPA/DHA, usually in the common 180/120 ratio.
It's advisable to take your Omega 3's with meals as the food can aid absorption as well as preventing/lmiting any reflux, although you can get good quality 'Reflux Free' fish oils nowadays. Price is indicative of quality and these are one supplement you do want to be getting as good as you can afford. They are pretty safe overall, just be careful of the interactions they may have with any mood-improving medications you may be taking as well as blood thinning agents such as Warfarin and even the common aspirin.
Other good food sources of Omega 3's are avacodo, cold water sea fish, extra virgin olive oil, peanut butter/coconut oil (be aware of the saturated fat content) and raw nuts ~ cashews are great as are brazil nuts. In terms of Omega 3 Vs flax oil, I would always go with the Omega 3 as being preferable. Generally women absorb flax oil better than men due to the fact that the body has to convert the oil before it can use it. Most people get enough Omega 6 & 9 through their diet, it's the Omega 3 that there tends to be more of a requirement for. Cheers MF.