Sorry to bump this but I'm interested in bootable PCI-E SSD drives, but didn't see if any verdict was reached about one?
For booting OS X, the answer is unfortunately No.

They can boot Windows in a MP (been tried successfully with an OCZ Revo IIRC).
There's just not enough of a market (such cards would need EFI32 for 2006 - 2007 MP's, EFI64 for 2008 and newer, or EBC based firmware <runs in either EFI32 or EFI64, which allows a "one size fits all" approach>).
I currently have four 750gb hard drives in the internal bays of my early 2008 Mac Pro, in a RAID-0 (striped) software array, but I currently have it as a single massive volume which is also my start-up disk.
A stripe set definitely improves sequential throughputs (moving large files), but they don't improve matters for random access (what OS/applications use relies on).
So I'm hoping to get a bootable SSD so I can have the OS and applications on the SSD, and user-folders/files on the big RAID drive. Then probably go all out and use a RAM-disk for /tmp and other hot-spots to avoid writing to the SSD too much (and since I have plenty of RAM for my needs so I can spare a bit).
In terms of SSD's, the boot capability is to do with the SATA controller they're connected to, not the drive itself. So as the included SATA ports (ICH = I/O Controller Hub) are bootable in your system, just attach an SSD to one of the ports.
Please note, as you're interested in dual booting via Boot camp, use HDD bays 1 - 4, as the ODD ports won't boot Windows without a hack, which is somewhat complicated. Much easier to just use an HDD bay.
I was hoping there'd be a good compatible PCI-E card for a single SSD, as while my lower optical bay is free I'm planning to get a Blu-Ray drive at some point as well, and installation for an SSD in that bay seems a bit unwieldy, plus with it using one of the spare SATA ports behind the main fan I'm not certain it would be bootable? I have an eSATA drive currently connected to one of those and although it's meant to be bootable OS X won't work from it.
Then something's got to go external.
On the surface, it would appear to be cheaper to place the SSD externally. Unfortunately however, a bootable card under OS X is by no means common or cheap (I've only found one, and it's $400 USD). So you'd want to take the existing HDD stripe set externally. The easiest and cheapest way to do this, is to get a 2 port eSATA card that supports Port Multipliers (
example), and a 4 bay Port Multiplier enclosure (
example kit = card + enclosure). But there is a speed penalty for this (PM chips top out at ~250MB/s).
For additional speed, you need a 1:1 disk to port ratio. To do this, you'd need a 4 port eSATA card (cheapest I know of that works in a MP =
here), and either a 4 bay enclosure that has 4x eSATA ports on the back (these are getting hard to find as most, if not all have been discontinued, but here's an
example), or 4x individual disk enclosures.
Either way, OS X can still operate these drives in a stripe set

(backup the data first, as they'll almost certainly be re-initialized to create a new set on the new location = existing data will be wiped out).
Also, I'm hoping to run a boot camp installation of windows from the SSD, which is something I just can't do at the moment with the RAID array.
You cannot do this with RAID at all. Neither software or hardware implementations support this.
Lastly, to weigh on the RAID part of the discussion; I'd definitely just start with software RAID; installing most cards is expensive, and a massive hassle for usually very small gain at best. The Software RAID is actually pretty damned fast as it is for opening large files, so the main area you'd see improvement from a hardware RAID would be in small-files (which the SSD should cover the bulk of anyway), so in my opinion it's just not worth it.
For stripe sets, there's no need to go with a RAID card at all (small throughput benefit at best), and it's costly.
It's other levels, particularly those that aren't supported by OS X, that you require the use of a proper hardware RAID card (OS X is only good for 0/1/10 and JBOD).