Plz advice on SLI'ing GTX570s

Hello,

I'm an owner of a semi-upgraded Dell 435MT with the following specs:
CPU i7 920 (2.6Ghz), 16GB DDR3 RAM, GTX570 Twin Frozr II, Corsair 750W PSU, Windows Vista 64bit Pro

I wanted to upgrade my LGA1366 stock Dell motherboard to a newer one in order to be able to OC my current CPU to like 3.0 Ghz (air cooled via Hyper212 CM), and possibly add another GTX570 to SLI and get performance similar to a single GTX680.

Here is GTX570 that I have now, but it's currently out of stock:
http://www.tigerdirect.com/applications/searchtools/ite...

Here is GTX570 that I want to buy, but not sure if it's the same or not:
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Here is the motherboard that I plan to SLI those babies on:
MSI X58A-GD45 LGA 1366 Intel X58 SATA 6Gb/s USB 3.0 ATX Intel Motherboard

I understand that the OC edition of GTX570 that I have is slightly better then the one on newegg, and I suspect that the OC version may "downclock itself" to make it compatible to the slower unit available through newegg. Will the both GTX570 fit on the motherboard in SLI mode? Also if yo have any thoughts whether this process is worth the time and $, versus just getting a new gtx680 and maybe get a better performance. I'm vigilant of the fact that I may loose out on some money, if I sell my gtx570 OC used and get gtx680. But if I keep my old GTX570 OC, I will still run into 120$ of expense for the new motherboard. I plan on using this setup mostly for gaming at 1080p resolutions. Please tell me if it's worth SLIing Thanks!