First off, I can guarantee that you won't need ECC RAM.
Second, in your situation, I wouldn't even bother with 2GB. The only difference I noticed when upgrading 1GB to 2GB is a little more responsiveness in Windows and opening programs. If you spend most of your time in WoW, then I don't think it's really worth your money to get an extra 1GB.
2 sticks vs 1 is only benficial if your mobo supports dual channel (and it does). Only even numbers of RAM sticks will work in dual channel (2 or 4).
You can mix and match RAM. I have come across some difficulties in doing so in the past, but nothing that is unfixable.
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