So far I have:

2x 7200.7's 80 GB in RAID0 & a single 7200.10 160 GB HDD. I could do with re-installing my OS (Vista Home Premium x64 - nLite OFC) because lots of things just aren't working. I could also do with more HDD space.

This gives me the chance to swap some things around:

I could buy another 7200.10 160 GB, and then RAID0 the two and use them for the OS

What are your thoughts on drives? Should I use one drive for the OS and one for programs? Or should I use the faster drives for both and then use the other drives for media storage?

Can I have 2 RAID0 arrays at the same time on my board (Asus P5K-E WiFi/AP - ICH9R southbridge)?

Should I get a single fast drive?

HALP!

Depends really on what you need fast disk access for. There's little point in having 320 GB in RAID0 if you only use it to store infrequently accessed stuff like archived downloads. That said, I'm not aware of limitations that would prevent you from having 2 RAID0 arrays, so you could
- keep the 2 7200.7's in RAID0 and
- get another 160 GB 7200.10's and put it in RAID0 with the other one

I don't see the point in striping the larger drives 'for the OS' unless you plan on doing other stuff like video editing on it, although even that would be better with a dedicated drive/array.

Some HD Tach Benchmarks:

7200.7's (RAID0):


7200.10:


Could I run two RAID arrays at once on my board? It's an Asus P5K-E WiFi/AP with the ICH9R southbridge.

I've got 2 74GB Raptors in RAID0 for my OS and games/programs and a 1TB Hitachi Deskstar for my main storage on this PC. In benchmarks the difference between the two different arrangements is huge, but in practise there's not really that much of a difference. Most of the time when you're reading from/writing to the drives they're not the limiting factor (e.g. when downloading files/when copying from CD/DVD, when transferring over LAN). The only time you can see a real difference is when you copy a file to the same drive. In that case the RAID0 is noticably quicker than the Deskstar. If you have enough money to be able to get a RAID setup then I'd say it's probably worth it. If you're not that bothered then you might be better saving money on the drives and spending it somewhere else (upgrading a different part of your system).

On the question of running multiple RAIDs from that board, I've got no idea, sorry. ASUS's support is really good though, so if you can't find an answer or don't get an answer here it'd be worth dropping them an email.

I've included some benchmarks from my system for comparison here. One from my Raptor RAID0, one from the single Hitachi Deskstar and one from one of my RAID5 network drives (taken from the PC that's hosting it though, as I can't get HDTach to run on network drives). The networked drives are on 100Mbit LAN at the moment anyway, so performance isn't great with them (should be going to gigabit at some point though).

@Jakg, from those benchmarks my conclusion is that you should go for SCSI.
But to be serious, I think running more than one RAID array on your MB should be fine. Will probably hurt performance a tiny bit, since software RAID uses the CPU, i.e. the performance of a software-based array is dependent on the CPU performance and load. If you can afford it, investing in a hardware based RAID card is well worth it.

I can get a second 160GB 7200.10 for less than £30 delivered so I think I will - I'll be putting the order in tommorow then getting all the stuff ready to reformat (sometime).

XFX Revo Raid Card - Costs about £7 from Scan, and if my orders over £20 I get free delivery and I have voucher for £6 off Scan.

Do i get one, or not?

Will it be any better than an onboard RAID controller? For £7, probably not.
http://forums.slizone.com/lofiversion/index.php?t5202.html