CSS Grid System for Forms (Multi-Column)

For future reference here is the final result with pixel perfect precision:

The CSS code:

._25 {
    width: 21%;
    display: inline;
    float: left;
    margin-left: 2%;
    margin-right: 2%;
}
._50 {
    width: 46%;
    display: inline;
    float: left;
    margin-left: 2%;
    margin-right: 2%;
}
._75 {
    width: 71%;
    display: inline;
    float: left;
    margin-left: 2%;
    margin-right: 2%;
}
._100 {
    width: 96%;
    display: inline;
    float: left;
    margin-left: 2%;
    margin-right: 2%;
}
label {
    width: 100%;
}
input {
    border: 1px solid #B3B3B3;
    width: 100%;
    -moz-border-radius: 3px;
}
textarea {
    border: 1px solid #B3B3B3;
    width: 100%;
    -moz-border-radius: 3px;
}
select {
    border: 1px solid #B3B3B3;
    width: 100%;
    -moz-border-radius: 3px;
}

And some sample HTML code:

<div class="_50">
    <p><label for="in_user">Username</label><input id="in_user" type="text" value=""/></p>
</div>

<div class="_50">
    <p><label for="in_pass">Password</label><input id="in_pass" type="text" value=""/></p>
</div>

Recently I've started using CSS grid systems and I find the whole process of designing a webpage much more simpler. Now I'm trying to stylize form elements but I'm having a really hard time making forms with columns, take the following example:

• div (width = 400px)
  • form
    • ul
      • li .half
        • label
        • input (should be 200px wide)
      • li .half
        • another label
        • another input (should also be 200px wide)

Basically I'm applying a class that has a width attribute of 50% but putting both inputs side by side makes the row to be bigger than 100% (400px) - I guess this is because of borders, margins and paddings.

Is there any CSS grid system that I can use to have multi-column forms while still making all the form elements have the same size (inputs, selects and textareas); eg. 1 input in 1 column should have 400px while 2 columns should have 200px each.

EDIT: Wufoo has some examples of what I'm trying to do but I'm too ignorant at CSS to understand all that code and I would appreciate if someone could give me some pointers.