if you pop open the case and read the label on the psu, there will be a grid with the numbers:
"+5V, +12V1, +12V2, -12V, -5V" etc across the top of the grid and corisponding numbers along the bottom like "8A, 10A, 10A, 1A, 1A" etc
depending on the type of PSU u have there will be a different number of 5V and 12V columns.
If you give us those numbers, we can tell you what kind of card you can support
It is most useful if you just list them off as though you were reading column by column:
i.e. +5V 5A, +12V1 10A, +12V2 10A ... etc