Intel has very good RAID drivers; relative to what is available on Windows. You also have the ability to activate 'write caching' option, which is dangerous (risk of filesystem corruption in case of crash/power outage) but can significantly speed up I/O in realistic cases of both reading and writing patterns. Its also mandatory to get any decent RAID5 write performance, but try to limit yourself to RAID0 or RAID1 if you can, as RAID5 is a much more complicated RAID level.
Only with a proper setup (not XP), and only if your applications allow all 6 HDDs to be used at once. Sequential transfers will use all 6, but with random I/O its more difficult to use all 6 drives. For example, things like booting will use only one drive during many parts of the booting process. All 6 drives will be used, but only one at a time so the performance is very close to single-drive performance. This is expressed in 'queue depth'. With a queue depth of 6, there would be 6 separate I/O requests outstanding, which can be sent to 6 individual drives to be processed; enhancing performance.
On windows though; i would say that more than 4 disks in RAID0 will get diminishing results.
If I were in your position right now, I'd be looking to "future-proof"
my core components with a SATA/6G and/or SAS/6G hardware RAID
controller, like Intel's RS2BL040 or RS2BL080 x8 PCI-E 2.0 controllers:
This will allow you much more growth potential (MAX HEADROOM)
as the entire industry ramps up to SATA/6G and SAS/6G HDDs,
a few of which are available now, but more importantly when
SATA/6G SSDs become more widely available, and less expensive.
Be sure to install such a controller in an x8 or x16 mechanical slot
to which the motherboard's chipset assigns a full x8 PCI-E 2.0 lanes.
The size of the edge connector on that controller may not be the
actual number of PCI-E lanes assigned to that slot.
If you're in the mood to investigate, Seagate have 3.5" Cheetah
and 2.5" Savvio HDDs that spin at 15,000 rpm and now support
an interface speed of 6 Gigabits per second e.g.:
MRFS, what good would SATA 6Gbps be for a harddrive that cannot even reach 150MB/s under the best of circumstances? I fail to see the point. For SSDs maybe, though even there the speed benefit comes from IOps not from sequential speeds.
If you want high sequential speeds, running RAID0 with lots of devices on the chipset SATA ports would be the best choice, as with 6 disks he would exceed 600MB/s throughput. I fail to see the need to focus on 6Gbps SATA, especially if he has to buy a separate add-on controller which adds latency opposed to chipset powered SATA ports. The next generation SATA chipsets (AMD 900 chipset series for example) will feature SATA 6Gbps ports. This is something you get 'for free' with the next generation of motherboards. Whether its useful is still unclear. HDDs will not benefit from it, for sure.