How to get table cells evenly spaced?

You can use CSS. One way is to set table-layout to fixed, which stops the table and its children from sizing according to their content. You can then set a fixed width on the relevant td elements. This should do the trick:

table.PerformanceTable {
    table-layout: fixed;
    width: 500px;
}
    table.PerformanceTable td.PerformanceCell {
        width: 75px;
    }

Suggestions for for tidying up? You don't need the cellpadding or cellspacing attributes, or the TableRow and TableHeader classes. You can cover those off in CSS:

table {
    /* cellspacing */
    border-collapse: collapse;
    border-spacing: 0;
}
th {
    /* This covers the th elements */
}
tr {
    /* This covers the tr elements */
}
th, td {
    /* cellpadding */
    padding: 0;
}

You should use a heading (e.g. <h2>) instead of <span class="Emphasis"> and a <p> or a table <caption> instead of the Source <span>. You wouldn't need the <br> elements either, because you'd be using proper block level elements.

You could always just set the width of each td to 100%/N columns.

<td width="x%"></td>

In your CSS file:

.TableHeader { width: 100px; }

This will set all of the td tags below each header to 100px. You can also add a width definition (in the markup) to each individual th tag, but the above solution would be easier.