Well, you didn't talk about money... So I'll i'll try to guide you in mid/high range.
Get a Z77 motherboard (overclocking friendly and needed for SLI/Crossefire). I don't know if all Z77 motherboards are SLI and Crossefire Certified (the chipset is capable of, but I do think they need Nvidia and AMD certification), so make sure it's clearly stated in the specs...
If you look in mid/high range Z77, look at MSI Z77A at least the GD45. I have the GD65 and it's a really nice motherboard. For a Gigabyte equivalent, look for UD4 or more. Asrock Extreme 4 is recommended by many forum users, so I suppose it's nice too. EVGA have only FTW edition which is, to me, overpriced.
For the CPU, if you can afford it, the 3570k is the best i5 CPU. Unlocked multi and performant.
With 8GB of DDR3 1600, 16GB if you can, this would be a nice upgrade.
I don't think you would need to change the RAM, maybe add another stick if there's an open slot. Get a Z77 chipset if you're overclocking, I would recommend the AsRock Z77 Extreme3 if you're on a budget. I agree with the i5-3570k, should last quite a while before becoming outdated.