It all depends on what you want to do. From the private message you sent me, you have an 8GB SSD
which is on the small side, especially if you want to run OS X, which you also mentioned.
A 16GB SSD
would be the smallest I could recommend (besides, you already have the next smallest size.) The RunCore models compatible with your Mini are here
. Another brand that is popular is SuperTalent, which you can find here
, but I have been reading more and more complaints about these and I hesitate to recommend them now.
I also looked at Newegg, but they were out of stock of all the SuperTalent drives and they don't carry RunCore.
For RAM, any of those listed here
You also asked about installing OS X. That is discussed extensively in the Mac OS X section of the forum. I will caution you that this is what I would consider an "advanced" topic and if you go into it blindly you will just get frustrated.