Hellhammer already nailed my point. You are part of the group Apple doesn't cater to.
However, a custom built PC will cater to you as you have all the expansions you want at native 2.0+ x16 PCIe's (and you get 2 not one) and depending on manufacturer multiple PCIe 1.x at x4 or x1 configurations for other expansions, such as RAID or USB3 add-ons.
Also, Mac Pros only support 4 HDDs, which makes using RAID 1+0 kind of silly since you use up all 4 bays just to do that. A Mac Pro can hold either RAID 1 or 0 depending on your need. However, like I said, a custom built can hold that 4-drive RAID 1+0 (or 0+1) and still have space for those SSDs.
Recall, using a Mac Pro only gives you only one x16 native PCIe slot and the rest are bottlenecked at x4 speeds. So having space for 2 HD5870s or 2 GTX480 isn't going to help as one of them will be bottlenecked. That or Apple eliminates the other expansion slots to run the extra bandwidth to the new PCIe at x16.
No, the Mac Pro gives you *2* x16 slots. Still not great, mind, considering the other 2 are bottlenecked at x4, but let's not make it worse than it is. You do, in fact, have space for 2xHD5870s, though you'd have to pull power from the HD leads in the optical bay for the second card because the 5870 uses 2x6-pin plugs, and there are only 2 provided from the mother board - a single plug card you use 2 of without the power issues.
They only advertise 1 x16 slot on the specs page because when shipped the other x16 lane is always occupied by a video card.
The x4 lanes aren't really all that bad now either, because, unlike my '08 model, all slots are now pci-e 2.0, which means double the throughput of pci-e 1.1, with 4 lanes of pci-e 2.0 @ 500MB/s per *lane* you're looking at 2GB/s aggregate on those x4 slots. That's higher than most fiber channel variants can hit right now and higher than most RAID solutions people are going to be using. If you need more, well, you're not the market for this machine, what can I say - though if you really need more than the back of napkin ~80TB of *local* space I see you would need to use to fully saturate the total lanes through RAID in this machine (assuming full theoretical bandwidth usage - not happening using RAID 5 or 6 - you have 2*(4*500MB/s) + 16*500MB/s = 12GB/s to work with in bandwidth, if at full SATA saturation you're looking at 300MB/s per drive you can drive 40 drives at full speed, or, at the moment, 80TB using 2TB drives) while leaving one vid card in.... I'd question why you need 80TB of local storage and why, when spending *that* much on disk you care about a single mac pro in the calculations.
Also, it's rather easy to use all 4 HD sleds for spinning disk and add SSDs to the optical bay using the extra 2 SATA plugs on the motherboard. I seem to recall people mentioning that in the current revisions ('09 and '10 models) there are already even SATA cables run up to the optical bays, which makes installation trivial, no harder than adding another optical drive.
