I have no doubt its fast, but with a RAID 5 you still see a significant improvement over a single drive. A PCI express RAID card is fine, but a regular pci raid card is a toilet and you are way better off with on-board.
IMO the best setup is what I'm doing. I stripe two raptors for the os and applications, setup a 2gb swap partition (fat 32) and run that all off a PCI-E Raid card. I then use the on-board RAID to do a RAID 5 array for storage using a 2+1 setup. You need a mighty power supply to support all that + optical, but its worth it. I also slipstreamed a copy of windows so I can reinstall with all my drivers when the stripped array fails, which is not an if, but a when. I have lost dozens of hard drives in the last 10 years (yes going back to my school computers and teen years!) and accept it as inevitability.
Also, benchmarking transfer speed alone isn't that useful, a reliable bench mark that actually a/b the performance of a RIP handling files from a RAID 0, 5 or a single drive would be more valuable. The processing overhead will probably make the speed garnered from a stripped array negligible. That combined with heinous reliability, and I don't think its a great choice for one's only storage.
I also use an additional webserver dedicated to offsite backup. Its a Free BSD box like my regular webserver, but its dedicated to just offsite. It appends every night at starting at 1am and is almost always done by the time I come in at 8am.