PCIe RAID cards seem absurdly expensive for mixed (or downright mediocre) reviews on the quality of said product.
The problem with most HDDs is that while they have improved in interface, reliability, storage, etc... Most still run at 7200 rpm. You can only get around a platter so fast.
There are a handful of 10000rpm that are pricey, like the Velociraptor 10k drives, and some SAS drives (special connector, meant more for server use). So when you RAID you will get more performance out of it, as compared to a 7200rpm drive. With gaming an SSD can be a blazing improvement or it can be little different than your HDD. The thing is it depends on if the game is reading levels, textures, maps, models, etc, or if it has to DECOMPRESS them as it reads them. In that case your CPU is the slow point, not the I/O to the HDD. In these games you don't notice any difference. In others it's amazing. I think the overall system responsiveness is pretty awesome, and even if the game doesn't take advantage of that it's great for other things supporting the game.
If/when I do a RAID it will be to help with read/write speeds when recording gameplay footage. I'm torn between a high capacity RAID or a RAM drive and recording shorter sessions. I really want to record more gameplay action but I think my storage is the problem. I saw a review where FRAPS was used on a maxed out Crysis2 (or some other game) at 1900p. When recording the FPS slumped to a pretty stuttery level. So they changed it to record FRAPS from a single dedicated HDD to a 2-SSD RAID 0 drive they set up, and it ran smooth as butter still maxed out, same game, same settings, but recording at 60 fps with no lag.
In that case FRAPs was being slowed down by read-write speeds. So to sum up a long answer: It's going to depend on what you want to use it for. IMO 4 disks minimum at RAID 1+0 is what you need for extra speed and redundancy, but isn't really "safe" as any HDD can fail without warning.
