Well, if you could get 4 sticks total of 512, that would be great. But, it's just fine to get one more stick of 512 and run them in dual channel.
Make sure to get a stick of RAM that is similar (speed, latency, etc) to the one you have now, to ensure dual channel compatibility.
Also, if for some reason you dont end up running your RAM in dual channel, dont worry too much about it, as dual channel is kinda overrated and only gives you about 1-5% performance increase.
Intel E6750...........PSN: ELBATROP
XFX nForce 650i Ultra
Patriot PC2-6400 8GB (4x2GB)
36GB WD Raptor
Samsung SyncMaster 931c
Samsung SyncMaster 750s
Windows 7 Home Premium 64