I don't know that I would see a noticable increase in speed by going to all cat 6.
You'll gain nothing going Cat6 and waste plenty of time and effort doing it.
I ran RAID10 on both motherboard-integrated controllers and PCI-E Areca controllers before. Hardware controller + RAID10 would have been a waste of money for media serving, so I stuck with onboard. RAID10 on software was also a waste of HDD space, so I stuck with RAID5. RAID is also a topic that gets confused a lot. RAID10 and RAID0+1 work differently.
RAID10, think about it as RAID1 + RAID0. It's a mirrored array (RAID1) of two striped arrays (RAID0). That means both drives from one side of the higher level mirrored array can fail and your data is intact.
RAID0+1, simply RAID0 + RAID1. It's a striped array (RAID0) of mirrored arrays (RAID1). That means one drive from each side of the higher level striped array can fail and your data is still intact.
In RAID5, you can only lose one drive, period. In RAID6, you have a doubly redundant set of parity bits, so you can lose two drives although this array type only exists on good controllers. No onboard chipset provides RAID6.
For comparisons since people love them, RAID10 and 0+1 will be pretty much identical in performance. In server environments with high workloads, RAID10 will typically rebuild faster from a single drive loss than RAID0+1. RAID0+1 will typically rebuild faster from a multiple drive failure. Sure it's arguable, but I'm just relating my observations.
RAID5 can nearly equal the performance of either above nested RAID type in reads (your streaming) but will lag them both in writes (saving your downloads, defragging, processing a PAR set, unpacking a RAR, swap file usage). It will dog a single drive although some people love to think not. It outruns my WD VelociRaptor150 in all but seek times, and my drives are not the fastest large capacity drives on the market. RAID5 also allows you to use an odd amount of discs, which is nice if you want to have 5 HDDs and one optical on a controller with 6 SATA ports for example, or 3 arrayed drives with 2 opticals and a fast (Raptor or SSD) boot drive.
With the MTBF of the current HDD market products, it's highly doubtful that under normal conditions you will lose two hard drives at the same time. Back in the IBM Deathstar days, it was highly likely because those drives were terrible. I killed literally 3 of the 4 I had in one system within a week of each other.
In this generation of quad core CPUs and 8GB physical RAM installations, most people don't want to believe that they are wasting the majority of their computing power. You are. Get over it. Your hardware can handle the processing load of the non-dedicated (I don't call it "software" because that's not what it is) RAID controller on your motherboard. I can guarantee you that regardless of what the performance tab in your Windows Task Manager says, you are not nearly using all of your available computing power.
Lastly, make sure your cables are up to snuff. A friend recently went mad over his lack of performance between his very powerful server and his WDTV Live. Turns out a bad cable fixed his problem. Another area of note is downloaded encodings. If the rip is bad, the nfo file means nothing. I've been able to stream 30GB BD rips with no problem and my NMT only has a 100Mb ethernet connection. By comparison I've had choppy 720P/DTS 6GB rips and the cause was the encoding.
Feel free to buy some Cat6, an Areca ARC-1260, and 16 2TB Caviar Blacks. It's your wallet.
