Edit: Wow, didn't realize this thread was so old. I'd apologize for necro-posting, but this was in the top 3 posts in this section before I posted, haha. I'll leave this post up for anyone who may find this thread through a search.
EV training is all about having control over your Pokémons' stats. It really depends on what you want to do with each 'mon; there are a few different schools of thought on EV training, which you should mix and match throughout your team:
- Many strategies max out Atk (or SpA) and speed and throw the remaining 4 into some defensive stat. This is a 252/252/4 spread, you see it a lot on sweepers. It would work well on your Swellow (4 HP / 252 Atk / 252 Spe).
- 252/248/8 is another common spread that you often see in bulky support 'mon that could use an extra Atk point or whatever.
- Bulky 'mon like Swampert are usually more well-rounded. If you plan on doing plenty of attacking with it you might want to max out SpA, and put defense and speed in the mid hundreds. If you want to go super bulky, you might want to go low 200s on the HP, Def and SpD split up based on what you expect to be facing (not sure if battle frontier has a higher spread of physical or special attackers off the top of my head but something for you to ponder).
- There are some situations where you may want to use EVs to make up for some of your mon's poorer stats, rather than bolstering their strong ones. This can surprise opponents, but should be used with caution as the meta for years has been to build extremely specialized Pokémon that are really good at what they do, and by doing this carelessly you may just end up bringing a mediocre 'mon to the match.
Whatever you do make sure that your EV investments are in a multiple of 4. For every 4 EVs, you get 1 additional stat point at level 100. The quotient is rounded down to the nearest whole number, so if you have 43 EVs in a stat, those last 3 are wasted. In gen 3 the game will let you put 255 EVs into a stat, understandable but evil of them; in gen 6 they capped EVs properly at 252. Note that the per-Pokémon max or 510 is also not divisible by 4; you will have 2 remaining when you're done training, and it doesn't matter where they go.