If your cpu and psu is sufficiently capable, SLI should help your frame rates.
That said, I suggest you wait to see how you actually do with your new monitor.
If you can, keep your old monitor connected; a second monitor is a most useful addition, and there should be little impact on gaming.
I use a second monitor to keep e-mail and some monitors open.
I am not much in favor of sli when a single good monitor will do the job. Some games do not do well with sli, and a single card will avoid microstuttering.
A 6970 should be a good improvement. Also look at a GTX570 which is in the same performance tier. Replacing with a Nvidia card will not require a driver removal/reinstallation procedure.
If you are looking 6 months out, you should see 28nm offerings from Nvidia and amd that should be a more attractive upgrade option.