3ware makes some excellent SATA II RAID controllers that are compatible with 4x PCI-E up to sixteen drives, and even a 1x PCI-E two drive controller. You could be striping your OS off of a pair of 37 gig raptors in a 1x slot for around $400, including the drives, with no CPU overhead. Those four way onboard RAID connectors are mighty convenient, but they do use the CPU for their processing, typically, and therefore cost performance.
Several manufacturers are now making 1x PCI-E video cards.
Creative's Audigy 3 sound card is rumored to be available PCI-E 1x.
This is the sort of hardware that's not easily found, even on awesome parts meta-search engines like pricewatch.com
I tend to run into it most when surfing mini-itx form factor speciality shop online retailers, they have a lot of obscure hardware due to very minimal expansion options on those mainboards. Be prepared to pay a hefty premium as compared to somewhere like NewEgg, or even as compared to a PCI device that accomplishes the same task. Strangely enough, I only seem to be able to find certain things in certain places, there are parts on websites in Europe especially that seem to adhere to this phenomenon.
Happy hunting. They really arent as useless as everyone claims, these slots...