Good solid board from a reliable maker.
the x16 slot can be used for almost anything that can use PCI-Express but is generally used for video cards. The x16 means it has 16 'lanes' of bandwidth to share and x4 means 4 lanes. The internal protocol used in PCI-E is much like the internet, it can be used to carry almost any type of data.
As you say you are new to this whole PCI-E thing, I would suggest you not worry too much about the details and suffice it to say the magic works here. Plug something in and it should Just Work.
You wrote that your old board has a Socket 754 onboard. A smile to my heart for that. I still have the last one I had, was using for a server, which retired after the NIC died. The successor sockets, (AM2, AM2+, AM3, AM3+) have all been quite fine and are reasonably backward and forward compatible, simply the differences being mostly memory related. My server is loaded on an ancient ASUS M2A-VM socket AM2 running 4GB DDR2 with an Athlon II 255.
ASUS released a BIOS update in 2010 which allowed some of the AM3 CPUs to work in this board. I love this type of support.
BTW the board specs are here:
This aside, you could go real cheap and get a nice Athlon II X2, X3 or X4 and a middle of the road card like a Radeon 7750 throw them into your board with a 2x2GB kit of cheap RAM and have your hair blown back. With the same board you could plonk down $500 or more get a monster video card and a FX8350 with some faster RAM and have an even faster system. I do remember my 754 box and yes it was pokey.
The FM2 socket systems are not quite as fast as the AM3+ but the bang for buck is very good, especially if you don't need a video card.