First let me say I don't do RAID-0. That said, yes you do have the two choices you stated.
I would put the RAID-0 on the ICH5R ports: the upper two on the mobo which are labelled "SATA1" and "SATA2". The reason for this is you will have a shorter and faster path to memory from here. There is a 266MB/s pipeline to the Northbridge from here so even though you won't reach those levels today, it leaves you lots of headroom.
The alternative, the Promise ports, will work also: the lower two labelled "SATA RAID1" and "SATA RAID2". However, since the Promise hangs off the PCI bus, your RAID traffic has to share PCI bus bandwidth with any expansion cards you might add, such as video-capture, and also your built-in firewire. You have a 133MB/s theoretical bus bandwidth through the PCI path. A pair of SATA running RAID-0, even WD's, can start to approach bus saturation, topping out at, I'd guess, around 110MB/s sustained. So there would not be much bandwidth left for other PCI cards or firewire. If you plan to use the firewire, then I especially would not go there, as it hangs off the same PCI bus as Promise.
However, from a data-flow standpoint, however, the Promise ports are great for your data-store, when your O/S is off ICH5R ports. This is because the data flows in and out through different paths, so you can get maximum throughput within the box. So stick a big 200GB disk out on the Promise: SATA or ATA-133, your choice, and keep your data out there (be sure to apply the Big Drive Enabler patch-- search this forum for the link).
Also see this thread, P4C800 E Raid question, where Randallizer gives actual Promise RAID0 numbers for some possible configs.
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