Need Upgrade Ram Recommendations for Mac Pro

I need to upgrade the memory in my Early 2009 Quad Core Mac Pro with FCP 7. Currently, there are three slots with 1 GB of Ram in each, the 4th is empty.

Online Apple store technician said this model doesn't need to have the ram equally balanced and suggested that I buy one 4 GB to put in the empty slot. Then I read this online:"The Nehalem Mac Pros use DDR3-1066 (PC3-8500) ECC DIMMs (not FB-DIMMs), no large heatsinks, with temperature sensors. There are 4 memory sockets (2009 Quad-core 2.66GHz, 2.93GHz, 3.33 GHz. They can implement dual channel or triple channel memory access modes. Fastest performance is using triple-channel mode, with three identical modules in sockets 1 through 3 and leaving socket 4 empty."

I need to spend as little as possible, but I'm also willing to spend as much as neccesary to have FCP working optimally.

I'd very much appreciate your advise. How much ram should I buy? Where is a good place to buy it?

Thanks,
Colleen

Hello I use Mr Memory online, you input your computer and they tell you exactly what you need also very cheap.

get in touch (or just read tech info on their site) with these folks: [eshop.macsales.com]

Best prices around, and they've been long recognized by folks here and elsewhere as being some of the best in the business (good warranties, support, pricing, etc)

I checked out both places and opted to Other World Computing (MacSales). They recommended a set of three, four gb of memory.

Thank you!
Colleen