The mac may be slower than your desktop, so unless you've stopped using it, I would leave the ssd there. Frys had a 32 gb ssd for only $35 after rebate last week. You'll see more deals like this at the end of the year and on black friday in the usa. If you want to upgrade something, try selling the old part first on craigslist, so you're not stuck with too many leftover parts. I'm a computer junky, and I'm trying hard to stop the compulsive spending. It's hard.
You won't be able to get more than $100 here for it. You should craigslist/ebay it or appreciate the boot time savings in your Macbook. There's no worthwhile upgrade for your desktop--all of o1die's tips were dead on.
If you offer $90 or less for it, I bet you'll get a bite here.
I guess a GTX 560Ti would be a worthwhile upgrade for you, but you'd probably get the money for that best by selling your 6790 and the SSD on ebay.