currently i have a XFX 9800 GTX+ video card, had it for a year and half.  now it's getting old.  i reckon the cheapest way to upgrade it is to buy another card and Sli it.
but, the problem is, do i have to buy one card which is exactly the same as my current one (i mean same brand, sample model, same chipset etc), or do i just need to buy one with 9800 GTX+ chipset, regardless of the brand etc?
help please~