Does ram need to match to work?

That is a more complicated question than you think. You CAN add another gigabyte module, however if your motherboard is using dual channel mode you will lose that ability (Im assuming you are using dual channel as you bought a matched pair). On Athlon XP systems you can add the gigabyte module no problem, but on athlon 64(socket939)/Intel P4 (865/845/SiS655) systems you need to use equal numbers of same sized modules in order to retain dual channel mode operation.

I am upgrading to an Athlon 64 system, so what you're saying is that in order to maintain my current ram running in dual channel mode I can only add another 512 stick if I've only got one free slot.

Does it need to be the same brand or can it just be the same speed?

Unfortunatly 3x512 modules will not work in dual channel mode, you have to either use 2x512 or 4x512. What you could do is get another 512MB module now and then get a forth 512MB module when you upgrade. Bear in mind that if you get a socket754 Athlon64, then you will not be able to use dual channel mode anyway.
What do you need so much RAM for?

Not necessarily, however dual channel mode can be very picky about RAM and may not work stabily with different brands.
I would very much recommend getting the same brand, and if possible modules, of RAM.

There are some motherboards that can run 3 modules in dual channel. Each memory stick is divided into 2 banks. So with 3 modules, you have 6 banks and it's the memory banks that are divided between the two memory channels.

Nvidia nforce2 ultra can run 3 sticks in dual channel, though this is for socket A only.

For Athlon 64, I believe you need 2 or 4 modules for dual channel. Using theoretical values, you can either run 1GB at 6400MB/s, 1.5 at 3200MB/s or 2GB at 3200MB/s.

Unless you really really really need 1.5GB or 2GB of memory, there is no way to justify adding a third memory stick.

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"Memory with lifetime warranty? So, whose lifetime is that?"
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AIM BrentUnitedMem

I'm getting a really good deal on this new pc, basically all the important parts (ASUS K8N-E Deluxe mobo, Athlon 64 processor and BFG GeForce 6800) are going to run $300.

I figured since I'm not spending all that much for basically a new pc that I would spend a couple extra bucks and get a little more ram. I basically use my pc as a gaming rig but would like to get into a little video editing.

In my experience you can't have too much ram when running games (especially EQ2). How much ram is enough for video editing? If I can get away with not spending any more money on ram I won't be heartbroken  

You should be ok with 1GB.

2GB is good for hardcore gamers and for rendering video/graphics. For editing, 1GB is fine. If you frequently edit with several programs at a time, 2GB will be good for you.

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"Memory with lifetime warranty? So, whose lifetime is that?"
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AIM BrentUnitedMem

I did video editing on my system with only 512MB, i can notice the difference now i have 1GB though. I dont find anything frustratingly slow with 1GB.