I would recommend some Western Digital Caviar Blue series, i'm using 2X1tb of these in raid 1 and no streaming problems, even recording with fraps is good (with half-size setting so 720p) also remember, raid 1 only speed up the load time, not the write time so a single caviar blue will be enough to record 720p footage without any big impact (as it have some impact even if it's playable)
For your wi-fi, make sure to have at least "N" and if possible N600 or N750 with the 5ghz band, it'll greatly increase the reliability of your connection as the 2.4ghz is now saturating and can have some "lag" as devices wait for other devices on the same frequencies to stop transmitting... (have trouble with my old N300 (2.4ghz only) as my neigboor are using the same frequencies and all the channels are overlapping (i'm in Montréal, canada so there's a lot of wi-fi networks and all the 2.4ghz channel are in use) At the beginning of my N600 Network, already i can see a huge improvement in webpages loading and minor lag issues (you know like 1/2 a second ones) have stop in online gaming due to the free and reliable network...
Unless than that, make sure to have a decent CPU and maybe a little more ram (streaming would surely use 1 or 2gb more than the game itself... 8gb is still enough tough...)