1. Unfortunately, I'm unfamiliar with sparkles card, but how much memory (VRAM) does each card have? If there is mis-matched memory, some of the available memory will not be used as the cards have to each hold the same data, as they take turns rendering a frame. Still, the cards will work fine, but perhaps with lower memory usage on one card. None-the-less, both cards will still scale the way they should.
SLI is flexible in that it doesn't matter what card goes in what slot, as long as your motherboard supports at least 2-way SLI. If you are wondering if it does, reply with you motherboard model number or name, and I can help you out with that.
Of course you can overclock the weaker card. MSI Afterburner is my choice, and the choice of many as it works with any vendors' cards. There are other software utilities, like GPU Tweak by ASUS. If your looking for them, just Google them, as well as read reviews for the utilities. Just make sure you check a setting in the utility that says: synchronise settings for similar graphics processors (found in settings).
2. my motherboard is asus p8z68 deluxe (sli ready)
3. how will i know how much to overclock the weaker card which i think is the sparkles, as its Core Clock: 1006 MHz which i think is that of the reference 680??
1. You will be able to use the full amount of VRAM specified for games, which is great!
2. Just install the cards in the first 2 slots. They will run at PCIe 8x respectively. Keep in mind that Ivy Bridge supports PCIe 3.0, which allows the bus to operate at double the bandwidth, although no real performance increase/loss is given.
3. If you synchronise the settings, both GPUs will run at the same speed, meaning that one GPU may down-clock itself to match the other. Now if you want to bring the lower card up to the performance, match the GPU clock. In this case: EVGA SC: 1084MHz, Sparkle 1006MHz (add 78MHz to the core clock in the overclocking utility, and you may need to add a tiny bit of voltage to be safe. Probably increase the Power Limit to 102% or add around 10mV to the core voltage). Either way, running both cards with no overclocking won't pose a treat to performance, as GPU Boost will compensate for the inequality.