How do you know you actually need to upgrade?? If you do, just stick with one set, like 3 x 4gb of ram. Even though intels ICH10 controller is really good, you could have problems when populating both sets of channels. And under no reason should you switch to dual channel. X58 is TRIPLE channel, why would you want to just throw 2 sticks in there. Unless you were talking about moving over to LGA1155 socket or something which I wouldn't suggest and would also be weird since you only wanted to upgrade your ram.
The easiest way to know IF you need more RAM is if there's no Free RAM listed in the Resource Monitor once you're running your Apps. Though 'Standby' can be used the (apps or GPU(s)) aren't running optimally; some sacrifices are being made. Example BF3 can suck-up, in total 1GB OS + 3.5GB Game + 3GB Shared Video = 7.5GB, to run optimally.