If your setup is like I think it is (single channel ram) then they shouldn't need to be the same type of ram. That's a new rule to apply to dual channel ram.
there is no 'type of setup' or 'dual channel RAM'
assuming your board and CPU support dual channel, then it basically boils down to:
2 exact sticks = dual channel
non exact sticks = single channel, simple as that.
I've mixed and matched plenty of RAM before without issues, so if you're just adding another 512mb stick, unless your motherboard manufacturer specifically states it doesn't like whatever brand you're buying, chances are it'll work.