As I understand it, you must install one of the modules to achieve true cross sell. I have the Cross Sell mod installed and am pleased with it's performance. The "We Also Recommend:" is part of it's packaging. You can see an example of it here.
The only downside to this mod is if you are adding it to an existing, product heavy store, as you need to go and manually make each & every "We Also Recommend:" connection manually. However, after that initial work is done, making those connections as you add new product is not a problem. (At least in my case where I'll add maybe 10 new product a week. Hundreds and it would be a time-suck) I am able to say that this module began to produce results immeadietly upon installation, increasing sales of those cross-marketed ietms.
If you have thousands of products instead of 10's or 100's, you may want to look into Better Together module. I installed it originally as some of it's bells & whistels sounded good (2-for-1 associated discounts, for example) but the logic behind making it happen gave me a headache and so I bailed. That however, may have just been the result of a bad day at the office for me and not a true reflection of the mod itself.
There is also the Cart Upsell/CrossSell on Shopping-Cart Page module, which I'm sure I looked into but can't for the life of me remember why I chose not to go with it.
If memory serves, the "Customers who bought this also purchased..." mod is built in to the admin. Try running a test purchase and see if it's already implemented. I know you can alter the number of products per row for the mod in admin/configuration/product info
Good luck!
