In my quest in building my first kick-ass PC, I'm wondering about controller cards. Do I need one for a SATA hard drive? What does it do? Is it necessary?

A SATA Controller Card is an expansion card that is utilised so that your motherboard can support SATA Hard Drives (as opposed to just standard Hard Drives).
However, SATA controllers are only really required for older moterboards which don't support SATA. As it stands, most modern motherboards will support SATA-II (300Mb/s). For example, my motherboard (ASUS P5N32-SLI premium) supports 6 x SATA II devices.
The only other time you may need a controller would be if that amount of connections was not enough.
As such, if you get a motherboard that has SATA connections, you will not need a SATA Controller Card.
Hope that helps!

Oh...well then I'm probably fine as I am planning on getting a pretty high-end mobo
( EVGA 122-CK-NF68-AR LGA 775 NVIDIA nForce 680i SLI ATX Intel Motherboard...as listed on Newegg).

So, no need for a controller card then.

Don't suppose I can attach my old serial ATA HD there, could I? Maybe with an adapter?

Well, that MoBo has 1 x PATA (ATA100) that will utilise up to two devices (ie. DVD Burner & HDD). That, or you can always use something like this:

http://www.newegg.com/Product/Product.asp?...N82E16822998001

that will plug into the back of your ATA HDD to allow you to connect it to a SATA port... ;)

Link to the mobo: http://www.newegg.com/Product/Product.asp?...N82E16813188009

Says it has 6 SATA II ports and 1 PATA port - no need for a controller card for SATA devices.

But, if you need more than 2 PATA devices, then the card that JPHarvey suggested will expand your ability to use PATA devices.