Well, when you talk about gaming, a huge part of it is the computer's graphics rendering performance, and that's done by the graphics card. Cheap computers tend to skimp on this component, so that's the main thing to watch out for.
Dragon Nest recommends a minimum of a GeForce 7600GT. To get an idea of how this stacks up compared to other cards, check out the table
here. For example, you'll notice that Intel's latest integrated graphics chip, the HD 3000, is 4 tiers below the minimum req, so that's a no-go. You want something at least a few tiers above the minimum for smooth gameplay.
RAM-wise, you want to have around 4GB. Anything on top of that won't hurt, but it won't benefit you either, so don't go for a pricier computer just because it has more RAM.
CPU-wise, it's hard to give any specific advice because it's a budget issue. And you didn't give a budget to work with!
So, what is your budget?