N1S, jbloggs is right that you won't get more speed from a controller as is inph.
However, that said, on nearly all motherboards for consumer and enthusiast use, The 'southbridge' or Controller Hub (ICHxR - Intel) type RAID implementations are budget, and about the slowest you can get. Also, on most implementations, will sit on a PCI bus (not great).
A fair bit of the work is done in the driver (ie main CPU) rather than offloaded in firmware on the raid controller.
This is why the more discs you added the lower the performance seemed to be, it's not he onboard software as such, but the way that the controller is implemented.
The CPU in your case is now loaded with extra drives, all hanging on the controller which doesn't in itself manage the data queing that a controller card would handle. (this is why consumer drive manyfaturers intergratede NCQ functions into their drives to try and overcome this).
Adaptec controllers are what i'd recommend, they're not cheap but its the weapon of choice for Dell, HP and IBM Enterprise Server solutions.
They run at x8 in the PCI-E slot, and have onboard ram themselves, so that with the native NCQ on the drive should give the perception of noticable speed. They also have drivers which are Vista x64 native.
Entry level there is the 4 port 2405 or the more expensive 3405 or 3805.
Their customer support is great, and you have someone that can hold a decent conversation at the other end rather than reading from a script.
Hope this helps.
