Nvidia's ShadowPlay will let you record 1080p 20 minute videos without even keeping the record process running. You just turn it on after you complete a 20 min gameplay segment you know you want, and it captures it lag free. ShadowPlay uses H.264 hardware built into the GPU.
You can also use it to record manually in the background if you want to do segments longer than 20. I mentioned ShadowPlay first because it's the most convenient and least time consuming. It also yields fairly small file sizes that are easy to store and quick to compress.
If you want something more feature rich that has way more setup options, including dual audio track recording, there's DxTory. Your spec will easily handle recording most any game at 1080p, but if you run into FPS issues at all with DxTory, you have the perfect setup to tweak it for zero lag.
With DxTory you can designate more than one HDD to write the capture file (Distribution Writing). It stores the video as "RawCap" files, sorta like RAID striping. After the file is captured you use the built in RawCapConvert tool to build the RawCap files into a single AVI file. Using DW requires setting output mode to RawCap instead of AVI, and using the DxTory codec.
That said, I highly recommend a fast 120-128GB SSD drive for OS and programs. With Windows, DxTory, and any compression software you may be using like Sony Vegas, etc, it will help speed up the process. All the better if you can afford a larger SSD to install the game/games you're capturing on.
[video="https://www.youtube.com/watch?v=W0KqsTa_HpM"][/video]