How important is it to have matching sets of RAM?

Question

How important is it to have matching sets of RAM? My workstation came with 6GBs of RAM. 3x2GB's

Its now time to install another 6Gbs. Is it absolutely crucial I order the same RAM?

My motherboard is a Asus P6TDeluxe v2.

Answer 1

Not really. There's a minor speed difference, but adding RAM will always make it faster, even if they don't match.

Source: My laptop and desktop had non-matching set of RAM.

EDIT: According to http://guides.macrumors.com/Buying_RAM (applicable for PC as well)

Dual Channel: many Apple machines use dual channel memory. In a few cases, this means that memory must be installed in matched pairs. Your computer's manual would say so if that's true. For the majority of cases, unpaired chips will work fine, but matched pairs will result in a slight speed increase (on the order of a few percent).

Then about the cheapest memory matter:

When buying memory, it is generally not advisable to go with the absolute cheapest stuff you can find. Low quality memory can and often will cause application crashes, total machine crashes, and data corruption. Do your homework on quality reputation up front, and be advised that while buying generic memory saves you money, you assume a greater risk of trouble later on.