My system currently is:
i7-950
Intel Extreme DX58S02 LGA 1366
16GB of ram at 533mhz
GTX 670
System drive is dual 256 SSD Raid 0 (occasionally imaged for restore if ever a failure)
3TB Work Drive 7200
3TB Render Drive 7200
1 90 GB cache/previews
Drobo S for photography only
When I was editing video from XH-A1 the 1080i was pretty taxing. I shoot with a XF100 now and most of the time I shoot 720p 60 and that edits way easier. The system has been running hot I just found recently when making mpeg files and I've had a few renders fail possibly to overheating. I just ordered a new cooler to help with that. Most of the work I do is Lightroom/Photoshop, but I have a lot of video to edit from all the dance recitals I've shot this spring. I mainly do a fast color corrector and not much else, but still like to dabble with personal projects with my D800 DSLR.
So maybe a 980, max ram with 1600Mhz, and a RAID-5 upgrade? I don't know if I'd gain much more at this point with spending more money on a Haswell when everything else is working pretty good...