Yes it's a dead socket but decent 939 dual core CPU's are still available.
I think an upgrade to an Opteron 180/185 or 4800+ would be noticeable. Even an FX60, if you want to OC, which is the same as the 185 with an unlocked multiplier.
Then again. Why not OC your CPU to run at 2600Mhz? I'd love to OC your CPU to 2700Mhz! That's another option.
Either way you still have options that would enable you to get the best of a decent graphics card. But you have to look at your RAM as well if you want super slick SLI.
One more question why have you not upgraded your single core Reminez?
Well i'm deciding whether to upgrade my CPU, RAM, etc or just do a full upgrade.